diff --git a/ApplicationLibCode/Application/Tools/RiaCurveMerger.inl b/ApplicationLibCode/Application/Tools/RiaCurveMerger.inl
index 87b21a9589..4110a16e7e 100644
--- a/ApplicationLibCode/Application/Tools/RiaCurveMerger.inl
+++ b/ApplicationLibCode/Application/Tools/RiaCurveMerger.inl
@@ -69,23 +69,29 @@ RiaCurveMerger<XValueType>::RiaCurveMerger( RiaCurveDefines::InterpolationMethod
 template <typename XValueType>
 void RiaCurveMerger<XValueType>::addCurveData( const std::vector<XValueType>& xValues, const std::vector<double>& yValues )
 {
-    CAF_ASSERT( xValues.size() == yValues.size() );
+    // X and Y can have different sizes when the data is read from file. This is a legitimate run-time condition, e.g. for an
+    // ongoing simulation where the last report step is not complete, or when depth values and result values come from different
+    // sources. computeInterpolatedValues() assumes that the two vectors have the same number of samples, so discard the trailing
+    // samples that have no counterpart. See https://github.com/OPM/ResInsight/issues/12810
+    const size_t sampleCount = std::min( xValues.size(), yValues.size() );
+    if ( sampleCount == 0 ) return;
 
-    if ( !xValues.empty() )
+    std::vector<XValueType> x( xValues.begin(), xValues.begin() + sampleCount );
+    std::vector<double>     y( yValues.begin(), yValues.begin() + sampleCount );
+
+    if ( m_originalValues.empty() )
     {
-        if ( m_originalValues.empty() )
-        {
-            m_isXValuesSharedBetweenCurves     = true;
-            m_isXValuesMonotonicallyIncreasing = isMonotonicallyIncreasing( xValues );
-        }
-        else
-        {
-            const auto& firstXValues = m_originalValues.front().first;
-            m_isXValuesSharedBetweenCurves &= std::equal( firstXValues.begin(), firstXValues.end(), xValues.begin(), xValues.end() );
-            m_isXValuesMonotonicallyIncreasing &= isMonotonicallyIncreasing( xValues );
-        }
-        m_originalValues.push_back( std::make_pair( xValues, yValues ) );
+        m_isXValuesSharedBetweenCurves     = true;
+        m_isXValuesMonotonicallyIncreasing = isMonotonicallyIncreasing( x );
     }
+    else
+    {
+        const auto& firstXValues = m_originalValues.front().first;
+        m_isXValuesSharedBetweenCurves &= std::equal( firstXValues.begin(), firstXValues.end(), x.begin(), x.end() );
+        m_isXValuesMonotonicallyIncreasing &= isMonotonicallyIncreasing( x );
+    }
+
+    m_originalValues.push_back( std::make_pair( std::move( x ), std::move( y ) ) );
 }
 
 //--------------------------------------------------------------------------------------------------
diff --git a/ApplicationLibCode/Application/Tools/RiaWellLogCurveMerger.cpp b/ApplicationLibCode/Application/Tools/RiaWellLogCurveMerger.cpp
index 8cf2d352a1..fb221d99d4 100644
--- a/ApplicationLibCode/Application/Tools/RiaWellLogCurveMerger.cpp
+++ b/ApplicationLibCode/Application/Tools/RiaWellLogCurveMerger.cpp
@@ -37,12 +37,14 @@ RiaWellLogCurveMerger::RiaWellLogCurveMerger()
 //--------------------------------------------------------------------------------------------------
 void RiaWellLogCurveMerger::addCurveData( const std::vector<double>& xValues, const std::vector<double>& yValues )
 {
-    CAF_ASSERT( xValues.size() == yValues.size() );
+    // Mismatching sizes is a legitimate run-time condition for data read from file. lookupYValue() discards the whole curve when
+    // the sizes differ, so truncate to the common sample count to keep the samples that do have a counterpart.
+    // See https://github.com/OPM/ResInsight/issues/12810
+    const size_t sampleCount = std::min( xValues.size(), yValues.size() );
+    if ( sampleCount == 0 ) return;
 
-    if ( !xValues.empty() )
-    {
-        m_originalValues.push_back( std::make_pair( xValues, yValues ) );
-    }
+    m_originalValues.push_back( std::make_pair( std::vector<double>( xValues.begin(), xValues.begin() + sampleCount ),
+                                                std::vector<double>( yValues.begin(), yValues.begin() + sampleCount ) ) );
 }
 
 //--------------------------------------------------------------------------------------------------
diff --git a/ApplicationLibCode/UnitTests/RigTimeCurveHistoryMerger-Test.cpp b/ApplicationLibCode/UnitTests/RigTimeCurveHistoryMerger-Test.cpp
index bc845fe409..e92f9f60f0 100644
--- a/ApplicationLibCode/UnitTests/RigTimeCurveHistoryMerger-Test.cpp
+++ b/ApplicationLibCode/UnitTests/RigTimeCurveHistoryMerger-Test.cpp
@@ -234,3 +234,56 @@ TEST( RiaTimeHistoryCurveMergerTest, SharedXValues )
     auto generatedYValuesB = interpolate.interpolatedYValuesForAllXValues( 1 );
     EXPECT_TRUE( std::equal( valuesB.begin(), valuesB.end(), generatedYValuesB.begin() ) );
 }
+
+//--------------------------------------------------------------------------------------------------
+/// A curve where the value count is smaller than the time step count must not read out of bounds.
+/// See https://github.com/OPM/ResInsight/issues/12810
+//--------------------------------------------------------------------------------------------------
+TEST( RiaTimeHistoryCurveMergerTest, FewerValuesThanTimeSteps )
+{
+    std::vector<time_t> timeSteps{ 1, 2, 3, 4, 5, 6, 7 };
+    std::vector<double> partialValues{ 1.0, 2.0, 3.0, 4.0 };
+
+    RiaTimeHistoryCurveMerger interpolate( RiaCurveDefines::InterpolationMethod::LINEAR );
+    interpolate.addCurveData( timeSteps, partialValues );
+    interpolate.computeInterpolatedValues( true );
+
+    // The trailing time steps without a value are discarded
+    EXPECT_EQ( partialValues.size(), interpolate.allXValues().size() );
+
+    auto generatedValues = interpolate.interpolatedYValuesForAllXValues( 0 );
+    ASSERT_EQ( partialValues.size(), generatedValues.size() );
+    EXPECT_TRUE( std::equal( partialValues.begin(), partialValues.end(), generatedValues.begin() ) );
+}
+
+//--------------------------------------------------------------------------------------------------
+/// Curves sharing time steps, but where one curve has fewer values, must not read out of bounds in
+/// the shared-X code path. See https://github.com/OPM/ResInsight/issues/12810
+//--------------------------------------------------------------------------------------------------
+TEST( RiaTimeHistoryCurveMergerTest, SharedXValuesWithFewerValuesInOneCurve )
+{
+    std::vector<time_t> timeSteps{ 1, 2, 3, 4, 5, 6, 7 };
+    std::vector<double> completeValues{ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0 };
+    std::vector<double> partialValues{ 10.0, 20.0, 30.0, 40.0 };
+
+    RiaTimeHistoryCurveMerger interpolate( RiaCurveDefines::InterpolationMethod::LINEAR );
+    interpolate.addCurveData( timeSteps, completeValues );
+    interpolate.addCurveData( timeSteps, partialValues );
+    interpolate.computeInterpolatedValues( true );
+
+    EXPECT_EQ( timeSteps.size(), interpolate.allXValues().size() );
+
+    auto generatedComplete = interpolate.interpolatedYValuesForAllXValues( 0 );
+    ASSERT_EQ( timeSteps.size(), generatedComplete.size() );
+    EXPECT_TRUE( std::equal( completeValues.begin(), completeValues.end(), generatedComplete.begin() ) );
+
+    auto generatedPartial = interpolate.interpolatedYValuesForAllXValues( 1 );
+    ASSERT_EQ( timeSteps.size(), generatedPartial.size() );
+    EXPECT_TRUE( std::equal( partialValues.begin(), partialValues.end(), generatedPartial.begin() ) );
+
+    // No data is available for the time steps beyond the last value
+    for ( size_t i = partialValues.size(); i < generatedPartial.size(); i++ )
+    {
+        EXPECT_EQ( HUGE_VAL, generatedPartial[i] );
+    }
+}