diff --git a/app/assets/javascripts/admin/controllers/admin-badge-preview.js.es6 b/app/assets/javascripts/admin/controllers/admin-badge-preview.js.es6
new file mode 100644
index 00000000000..e141dd24e40
--- /dev/null
+++ b/app/assets/javascripts/admin/controllers/admin-badge-preview.js.es6
@@ -0,0 +1,49 @@
+export default Ember.Controller.extend({
+  needs: ['modal'],
+
+  sample: Em.computed.alias('model.sample'),
+  errors: Em.computed.alias('model.errors'),
+  count: Em.computed.alias('model.grant_count'),
+
+  count_warning: function() {
+    if (this.get('count') <= 10) {
+      return this.get('sample.length') !== this.get('count');
+    } else {
+      return this.get('sample.length') !== 10;
+    }
+  }.property('count', 'sample.length'),
+
+  has_query_plan: function() {
+    return !!this.get('model.query_plan');
+  }.property('model.query_plan'),
+
+  query_plan_html: function() {
+    var raw = this.get('model.query_plan'),
+        returned = "<pre>";
+
+    _.each(raw, function(linehash) {
+      returned += Handlebars.Utils.escapeExpression(linehash["QUERY PLAN"]);
+      returned += "<br>";
+    });
+
+    returned += "</pre>";
+    return returned;
+  }.property('model.query_plan'),
+
+  processed_sample: Ember.computed.map('model.sample', function(grant) {
+    var i18nKey = 'admin.badges.preview.grant.with',
+        i18nParams = { username: Handlebars.Utils.escapeExpression(grant.username) };
+
+    if (grant.post_id) {
+      i18nKey += "_post";
+      i18nParams.link = "<a href='/p/" + grant.post_id + "' data-auto-route='true'>" + Handlebars.Utils.escapeExpression(grant.title) + "</a>";
+    }
+
+    if (grant.granted_at) {
+      i18nKey += "_time";
+      i18nParams.time = Handlebars.Utils.escapeExpression(moment(grant.granted_at).format(I18n.t('dates.long_with_year')));
+    }
+
+    return I18n.t(i18nKey, i18nParams);
+  })
+});
diff --git a/app/assets/javascripts/admin/controllers/admin-badges.js.es6 b/app/assets/javascripts/admin/controllers/admin-badges.js.es6
index 199c7eab1ee..c679db8402b 100644
--- a/app/assets/javascripts/admin/controllers/admin-badges.js.es6
+++ b/app/assets/javascripts/admin/controllers/admin-badges.js.es6
@@ -78,21 +78,6 @@ export default Ember.ArrayController.extend({
 
   actions: {
 
-    preview: function(badge) {
-      // TODO wire modal and localize
-      Discourse.ajax('/admin/badges/preview.json', {
-        method: 'post',
-        data: {sql: badge.query, target_posts: !!badge.target_posts}
-      }).then(function(json){
-        if(json.error){
-          bootbox.alert(json.error);
-        } else {
-          bootbox.alert(json.grant_count + " badges to be assigned");
-        }
-      });
-
-    },
-
     /**
       Create a new badge and select it.
 
@@ -128,16 +113,21 @@ export default Ember.ArrayController.extend({
                      'enabled', 'show_posts',
                      'target_posts', 'name', 'description',
                      'icon', 'query', 'badge_grouping_id',
-                     'trigger', 'badge_type_id'];
+                     'trigger', 'badge_type_id'],
+            self = this;
 
-        if(this.get('selectedItem.system')){
+        if (this.get('selectedItem.system')){
           var protectedFields = this.get('protectedSystemFields');
           fields = _.filter(fields, function(f){
             return !_.include(protectedFields,f);
           });
         }
 
-        this.get('selectedItem').save(fields);
+        this.get('selectedItem').save(fields).catch(function(error) {
+          // this shows the admin-badge-preview modal with the error
+          // kinda weird, but it consolidates the display logic for badge errors
+          self.send('saveError', error);
+        });
       }
     },
 
diff --git a/app/assets/javascripts/admin/routes/admin_badges_route.js b/app/assets/javascripts/admin/routes/admin_badges_route.js
index ba8fb4f9c57..4ef15173d36 100644
--- a/app/assets/javascripts/admin/routes/admin_badges_route.js
+++ b/app/assets/javascripts/admin/routes/admin_badges_route.js
@@ -15,8 +15,39 @@ Discourse.AdminBadgesRoute = Discourse.Route.extend({
   },
 
   actions: {
-    editGroupings: function(model){
+    editGroupings: function(model) {
       Discourse.Route.showModal(this, 'admin_edit_badge_groupings', model);
+    },
+
+    saveError: function(jqXhr) {
+      if (jqXhr.status === 422) {
+        Discourse.Route.showModal(this, 'admin_badge_preview', jqXhr.responseJSON);
+      } else {
+        Em.Logger.error(jqXhr);
+        bootbox.alert(I18n.t('errors.description.unknown'));
+      }
+    },
+
+    preview: function(badge, explain) {
+      var self = this;
+
+      badge.set('preview_loading', true);
+      Discourse.ajax('/admin/badges/preview.json', {
+        method: 'post',
+        data: {
+          sql: badge.query,
+          target_posts: !!badge.target_posts,
+          trigger: badge.trigger,
+          explain: explain
+        }
+      }).then(function(json) {
+        badge.set('preview_loading', false);
+        Discourse.Route.showModal(self, 'admin_badge_preview', json);
+      }).catch(function(error) {
+        badge.set('preview_loading', false);
+        Em.Logger.error(error);
+        bootbox.alert("Network error");
+      });
     }
   }
 
diff --git a/app/assets/javascripts/admin/templates/badges.js.handlebars b/app/assets/javascripts/admin/templates/badges.js.handlebars
index 57d7ae71008..1f0a42725e2 100644
--- a/app/assets/javascripts/admin/templates/badges.js.handlebars
+++ b/app/assets/javascripts/admin/templates/badges.js.handlebars
@@ -76,7 +76,12 @@
 
       {{#if hasQuery}}
 
-      <a href="/admin/badges/preview" {{action preview this}}>{{i18n admin.badges.preview}}</a>
+      <a href {{action preview this "false"}}>{{i18n admin.badges.preview.link_text}}</a>
+      |
+      <a href {{action preview this "true"}}>{{i18n admin.badges.preview.plan_text}}</a>
+      {{#if preview_loading}}
+        {{i18n loading}}...
+      {{/if}}
 
       <div>
         <span>
diff --git a/app/assets/javascripts/admin/templates/modal/admin_badge_preview.js.handlebars b/app/assets/javascripts/admin/templates/modal/admin_badge_preview.js.handlebars
new file mode 100644
index 00000000000..12d78af55d1
--- /dev/null
+++ b/app/assets/javascripts/admin/templates/modal/admin_badge_preview.js.handlebars
@@ -0,0 +1,44 @@
+<div class="badge-query-preview">
+  {{#if errors}}
+    <p class="error-header">{{i18n admin.badges.preview.sql_error_header}}</p>
+
+    <div class="badge-errors">
+      {{errors}}
+    </div>
+    <!--
+    TODO we want some help pages for this, link to those instead
+    <p>
+      {{i18n admin.badges.preview.error_help}}
+    </p>
+    <ul>
+      <li><a href="https://meta.discourse.org/t/triggered-custom-badge-queries/19336">https://meta.discourse.org/t/triggered-custom-badge-queries/19336</a></li>
+    </ul>
+    -->
+  {{else}}
+    <p class="grant-count">{{{i18n admin.badges.preview.grant_count count=count}}}</p>
+
+    {{#if count_warning}}
+      <div class="count-warning">
+        <p class="heading"><i class="fa fa-warning"></i> {{i18n admin.badges.preview.bad_count_warning.header}}</p>
+        <p class="body">{{i18n admin.badges.preview.bad_count_warning.text}}</p>
+      </div>
+    {{/if}}
+
+    {{#if sample}}
+      <p class="sample">
+        {{i18n admin.badges.preview.sample}}
+      </p>
+      <ul>
+        {{#each html in processed_sample}}
+          <li>{{{html}}}</li>
+        {{/each}}
+      </ul>
+    {{/if}}
+
+    {{#if has_query_plan}}
+        <div class="badge-query-plan">
+          {{{query_plan_html}}}
+        </div>
+    {{/if}}
+  {{/if}}
+</div>
diff --git a/app/assets/javascripts/admin/views/modals/admin_badge_preview_view.js b/app/assets/javascripts/admin/views/modals/admin_badge_preview_view.js
new file mode 100644
index 00000000000..0d890b3d663
--- /dev/null
+++ b/app/assets/javascripts/admin/views/modals/admin_badge_preview_view.js
@@ -0,0 +1,5 @@
+
+Discourse.AdminBadgePreviewView = Discourse.ModalBodyView.extend({
+  templateName: 'admin/templates/modal/admin_badge_preview',
+  title: I18n.t('admin.badges.preview.modal_title')
+});
diff --git a/app/assets/javascripts/discourse/models/badge.js b/app/assets/javascripts/discourse/models/badge.js
index 3752bf5a57e..43ca8d5e750 100644
--- a/app/assets/javascripts/discourse/models/badge.js
+++ b/app/assets/javascripts/discourse/models/badge.js
@@ -146,10 +146,10 @@ Discourse.Badge = Discourse.Model.extend({
       self.set('savingStatus', I18n.t('saved'));
       self.set('saving', false);
       return self;
-    }, function(error){
-      self.set('savingStatus', '');
+    }).catch(function(error) {
+      self.set('savingStatus', I18n.t('failed'));
       self.set('saving', false);
-      bootbox.alert(error.responseText);
+      throw error;
     });
   },
 
diff --git a/app/assets/stylesheets/common/admin/admin_base.scss b/app/assets/stylesheets/common/admin/admin_base.scss
index 28342d4b210..748c21ff8a8 100644
--- a/app/assets/stylesheets/common/admin/admin_base.scss
+++ b/app/assets/stylesheets/common/admin/admin_base.scss
@@ -419,6 +419,38 @@ section.details {
   }
 }
 
+.badge-query-preview {
+  .grant-count, .sample, .error-header {
+    margin-left: 10px;
+  }
+
+  .badge-query-plan, .badge-errors {
+    padding: 4px;
+    background-color: scale-color-diff();
+  }
+
+  .badge-query-plan {
+    font-size: 80%;
+  }
+  .badge-errors {
+    font-family: monospace;
+  }
+
+  .count-warning {
+    background-color: dark-light-diff(rgba($danger,.7), $secondary, 50%, -60%);
+    margin: 0 0 7px 0;
+    padding: 10px 20px;
+
+    p {
+      margin: 0;
+    }
+    .heading {
+      color: $danger;
+      font-weight: bold;
+    }
+  }
+}
+
 // Customise area
 .customize {
   .nav.nav-pills {
diff --git a/app/controllers/admin/badges_controller.rb b/app/controllers/admin/badges_controller.rb
index cd5170ee98d..00a47fc6705 100644
--- a/app/controllers/admin/badges_controller.rb
+++ b/app/controllers/admin/badges_controller.rb
@@ -14,7 +14,10 @@ class Admin::BadgesController < Admin::AdminController
   end
 
   def preview
-    render json: BadgeGranter.preview(params[:sql], target_posts: params[:target_posts] == "true")
+    render json: BadgeGranter.preview(params[:sql],
+                                      target_posts: params[:target_posts] == "true",
+                                      explain: params[:explain] == "true",
+                                      trigger: params[:trigger].to_i)
   end
 
   def badge_types
@@ -53,9 +56,28 @@ class Admin::BadgesController < Admin::AdminController
 
   def update
     badge = find_badge
-    update_badge_from_params(badge)
-    badge.save!
-    render_serialized(badge, BadgeSerializer, root: "badge")
+
+    error = nil
+    Badge.transaction do
+      update_badge_from_params(badge)
+
+      # Perform checks to prevent bad queries
+      begin
+        BadgeGranter.contract_checks!(badge.query, { target_posts: badge.target_posts, trigger: badge.trigger })
+      rescue => e
+        # noinspection RubyUnusedLocalVariable
+        error = e.message
+        raise ActiveRecord::Rollback
+      end
+
+      badge.save!
+    end
+
+    if error
+      render_json_error error
+    else
+      render_serialized(badge, BadgeSerializer, root: "badge")
+    end
   end
 
   def destroy
diff --git a/app/models/badge.rb b/app/models/badge.rb
index 5c398ff5ff3..a31580231df 100644
--- a/app/models/badge.rb
+++ b/app/models/badge.rb
@@ -31,6 +31,18 @@ class Badge < ActiveRecord::Base
     PostRevision = 2
     TrustLevelChange = 4
     UserChange = 8
+
+    def self.is_none?(trigger)
+      [0].include? trigger
+    end
+
+    def self.uses_user_ids?(trigger)
+      [4, 8].include? trigger
+    end
+
+    def self.uses_post_ids?(trigger)
+      [1, 2].include? trigger
+    end
   end
 
   module Queries
diff --git a/app/services/badge_granter.rb b/app/services/badge_granter.rb
index ade8d503fa7..46155e79cea 100644
--- a/app/services/badge_granter.rb
+++ b/app/services/badge_granter.rb
@@ -132,9 +132,38 @@ class BadgeGranter
     "badge_queue".freeze
   end
 
+  # Options:
+  #   :target_posts - whether the badge targets posts
+  #   :trigger - the Badge::Trigger id
+  def self.contract_checks!(sql, opts = {})
+    return unless sql.present?
+    if Badge::Trigger.uses_post_ids?(opts[:trigger])
+      raise "Contract violation:\nQuery triggers on posts, but does not reference the ':post_ids' array" unless sql.match /:post_ids/
+    end
+    if Badge::Trigger.uses_user_ids?(opts[:trigger])
+      raise "Contract violation:\nQuery triggers on users, but does not reference the ':user_ids' array" unless sql.match /:user_ids/
+    end
+    if opts[:trigger] && !Badge::Trigger.is_none?(opts[:trigger])
+      raise "Contract violation:\nQuery is triggered, but does not reference the ':backfill' parameter.\n(Hint: if :backfill is TRUE, you should ignore the :post_ids/:user_ids)" unless sql.match /:backfill/
+    end
+
+    # TODO these three conditions have a lot of false negatives
+    if opts[:target_posts]
+      raise "Contract violation:\nQuery targets posts, but does not return a 'post_id' column" unless sql.match /post_id/
+    end
+    raise "Contract violation:\nQuery does not return a 'user_id' column" unless sql.match /user_id/
+    raise "Contract violation:\nQuery does not return a 'granted_at' column" unless sql.match /granted_at/
+  end
+
+  # Options:
+  #   :target_posts - whether the badge targets posts
+  #   :trigger - the Badge::Trigger id
+  #   :explain - return the EXPLAIN query
   def self.preview(sql, opts = {})
     params = {user_ids: [], post_ids: [], backfill: true}
 
+    BadgeGranter.contract_checks!(sql, opts)
+
     # hack to allow for params, otherwise sanitizer will trigger sprintf
     count_sql = "SELECT COUNT(*) count FROM (#{sql}) q WHERE :backfill = :backfill"
     grant_count = SqlBuilder.map_exec(OpenStruct, count_sql, params).first.count.to_i
@@ -156,23 +185,34 @@ class BadgeGranter
     LIMIT 10"
      end
 
+    query_plan = nil
+    query_plan = ActiveRecord::Base.exec_sql("EXPLAIN #{sql}", params) if opts[:explain]
+
     sample = SqlBuilder.map_exec(OpenStruct, grants_sql, params).map(&:to_h)
 
-    {grant_count: grant_count, sample: sample}
+    sample.each do |result|
+      raise "Query returned a non-existent user ID:\n#{result[:id]}" unless User.find(result[:id]).present?
+      raise "Query did not return a badge grant time\n(Try using 'current_timestamp granted_at')" unless result[:granted_at]
+      if opts[:target_posts]
+        raise "Query did not return a post ID" unless result[:post_id]
+        raise "Query returned a non-existent post ID:\n#{result[:post_id]}" unless Post.find(result[:post_id]).present?
+      end
+    end
+
+    {grant_count: grant_count, sample: sample, query_plan: query_plan}
   rescue => e
-    {error: e.to_s}
+    {errors: e.message}
   end
 
   MAX_ITEMS_FOR_DELTA = 200
   def self.backfill(badge, opts=nil)
     return unless badge.query.present? && badge.enabled
 
+    post_ids = user_ids = nil
+
     post_ids = opts[:post_ids] if opts
     user_ids = opts[:user_ids] if opts
 
-    post_ids = nil unless post_ids.present?
-    user_ids = nil unless user_ids.present?
-
     # safeguard fall back to full backfill if more than 200
     if (post_ids && post_ids.length > MAX_ITEMS_FOR_DELTA) ||
        (user_ids && user_ids.length > MAX_ITEMS_FOR_DELTA)
@@ -180,6 +220,9 @@ class BadgeGranter
       user_ids = nil
     end
 
+    post_ids = nil unless post_ids.present?
+    user_ids = nil unless user_ids.present?
+
     full_backfill = !user_ids && !post_ids
 
     post_clause = badge.target_posts ? "AND (q.post_id = ub.post_id OR NOT :multiple_grant)" : ""
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index 967de72cf3d..0c7ddc673b0 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -204,6 +204,7 @@ en:
     disable: "Disable"
     undo: "Undo"
     revert: "Revert"
+    failed: "Failed"
 
     banner:
       close: "Dismiss this banner."
@@ -2024,7 +2025,6 @@ en:
         target_posts: Query targets posts
         auto_revoke: Run revocation query daily
         show_posts: Show post granting badge on badge page
-        preview: Preview badge
         trigger: Trigger
         trigger_type:
           none: "Update daily"
@@ -2032,6 +2032,25 @@ en:
           post_revision: "When a user edits or creates a post"
           trust_level_change: "When a user changes trust level"
           user_change: "When a user is edited or created"
+        preview:
+          link_text: "Preview granted badges"
+          plan_text: "Preview with query plan"
+          modal_title: "Badge Query Preview"
+          sql_error_header: "There was an error with the query."
+          error_help: "See the following links for help with badge queries."
+          bad_count_warning:
+            header: "WARNING!"
+            text: "There are missing grant samples. This happens when the badge query returns user IDs or post IDs that do not exist. This may cause unexpected results later on - please double-check your query."
+          grant_count:
+            zero: "No badges to be assigned."
+            one: "<b>1</b> badge to be assigned."
+            other: "<b>%{count}</b> badges to be assigned."
+          sample: "Sample:"
+          grant:
+            with: <span class="username">%{username}</span>
+            with_post: <span class="username">%{username}</span> for post in %{link}
+            with_post_time: <span class="username">%{username}</span> for post in %{link} at <span class="time">%{time}</span>
+            with_time: <span class="username">%{username}</span> at <span class="time">%{time}</span>
 
     lightbox:
       download: "download"