Use after_commit since after_save runs within the transaction as well.

Gemfile

@@ -120,6 +120,8 @@ group :test do
   gem 'fakeweb', '~> 1.3.0', require: false
   gem 'minitest', require: false
   gem 'timecop'
+  # TODO: Remove once we upgrade to Rails 5.
+  gem 'test_after_commit'
 end
 
 group :test, :development do

Gemfile.lock

@@ -359,6 +359,8 @@ GEM
       activesupport (>= 4.0)
       sprockets (>= 3.0.0)
     stackprof (0.2.10)
+    test_after_commit (1.1.0)
+      activerecord (>= 3.2)
     thor (0.19.1)
     thread_safe (0.3.5)
     tilt (2.0.5)
@@ -470,6 +472,7 @@ DEPENDENCIES
   sinatra
   spork-rails
   stackprof
+  test_after_commit
   thor
   timecop
   uglifier

app/models/user.rb

@@ -96,7 +96,6 @@ class User < ActiveRecord::Base
 
   before_save :update_username_lower
   before_save :ensure_password_is_hashed
-  before_save :setup_trigger_user_created_event
 
   after_save :expire_tokens_if_password_changed
   after_save :automatic_group_membership
@@ -105,7 +104,7 @@ class User < ActiveRecord::Base
   after_save :badge_grant
   after_save :expire_old_email_tokens
   after_save :index_search
-  after_save :trigger_user_created_event
+  after_commit :trigger_user_created_event, on: :create
 
   before_destroy do
     # These tables don't have primary keys, so destroying them with activerecord is tricky:
@@ -1078,13 +1077,8 @@ class User < ActiveRecord::Base
     end
   end
 
-  def setup_trigger_user_created_event
-    @trigger_user_created_event = self.new_record?
-    true
-  end
-
   def trigger_user_created_event
-    DiscourseEvent.trigger(:user_created, self) if @trigger_user_created_event
+    DiscourseEvent.trigger(:user_created, self)
     true
   end