diff --git a/smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ChangedEnumTrait.java b/smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ChangedEnumTrait.java
index 8ccc9ad9f0c..6bffe92ef85 100644
--- a/smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ChangedEnumTrait.java
+++ b/smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ChangedEnumTrait.java
@@ -29,8 +29,10 @@
 import software.amazon.smithy.utils.Pair;
 
 /**
- * Emits a NOTE when a new enum value is added, emits an ERROR when an
- * enum value is removed, and emits an ERROR when an enum name changes.
+ * Emits a NOTE when a new enum value is appended, emits an ERROR when an
+ * enum value is removed, emits an ERROR when an enum name changes, and
+ * emits an ERROR when a new enum value is inserted before the end of the
+ * list of existing values.
  */
 public final class ChangedEnumTrait extends AbstractDiffEvaluator {
     @Override
@@ -46,6 +48,7 @@ private List<ValidationEvent> validateEnum(ChangedShape<Shape> change, Pair<Enum
         EnumTrait oldTrait = trait.getLeft();
         EnumTrait newTrait = trait.getRight();
         List<ValidationEvent> events = new ArrayList<>();
+        int oldEndPosition = oldTrait.getValues().size() - 1;
 
         for (EnumDefinition definition : oldTrait.getValues()) {
             Optional<EnumDefinition> maybeNewValue = newTrait.getValues().stream()
@@ -55,6 +58,7 @@ private List<ValidationEvent> validateEnum(ChangedShape<Shape> change, Pair<Enum
             if (!maybeNewValue.isPresent()) {
                 events.add(error(change.getNewShape(), String.format(
                         "Enum value `%s` was removed", definition.getValue())));
+                oldEndPosition--;
             } else {
                 EnumDefinition newValue = maybeNewValue.get();
                 if (!newValue.getName().equals(definition.getName())) {
@@ -67,11 +71,21 @@ private List<ValidationEvent> validateEnum(ChangedShape<Shape> change, Pair<Enum
             }
         }
 
+        int newPosition = 0;
         for (EnumDefinition definition : newTrait.getValues()) {
             if (!oldTrait.getEnumDefinitionValues().contains(definition.getValue())) {
-                events.add(note(change.getNewShape(), String.format(
-                        "Enum value `%s` was added", definition.getValue())));
+                if (newPosition <= oldEndPosition) {
+                    events.add(error(change.getNewShape(), String.format(
+                            "Enum value `%s` was inserted before the end of the list of existing values. This can "
+                                    + "cause compatibility issues when ordinal values are used for iteration, "
+                                    + "serialization, etc.", definition.getValue())));
+                } else {
+                    events.add(note(change.getNewShape(), String.format(
+                            "Enum value `%s` was appended", definition.getValue())));
+                }
             }
+
+            newPosition++;
         }
 
         return events;
diff --git a/smithy-diff/src/test/java/software/amazon/smithy/diff/evaluators/ChangedEnumTraitTest.java b/smithy-diff/src/test/java/software/amazon/smithy/diff/evaluators/ChangedEnumTraitTest.java
index e75690686ce..3dba083834d 100644
--- a/smithy-diff/src/test/java/software/amazon/smithy/diff/evaluators/ChangedEnumTraitTest.java
+++ b/smithy-diff/src/test/java/software/amazon/smithy/diff/evaluators/ChangedEnumTraitTest.java
@@ -17,6 +17,7 @@
 
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.stringContainsInOrder;
 
 import java.util.List;
 import org.junit.jupiter.api.Test;
@@ -30,7 +31,7 @@
 
 public class ChangedEnumTraitTest {
     @Test
-    public void detectsAddedEnums() {
+    public void detectsAppendedEnums() {
         StringShape s1 = StringShape.builder()
                 .id("foo.baz#Baz")
                 .addTrait(EnumTrait.builder()
@@ -96,4 +97,61 @@ public void detectsRenamedEnums() {
         assertThat(TestHelper.findEvents(events, "ChangedEnumTrait").size(), equalTo(1));
         assertThat(TestHelper.findEvents(events, Severity.ERROR).size(), equalTo(1));
     }
+
+    @Test
+    public void detectsInsertedEnums() {
+        StringShape s1 = StringShape.builder()
+                .id("foo.baz#Baz")
+                .addTrait(EnumTrait.builder()
+                        .addEnum(EnumDefinition.builder().value("foo").build())
+                        .build())
+                .build();
+        StringShape s2 = StringShape.builder()
+                .id("foo.baz#Baz")
+                .addTrait(EnumTrait.builder()
+                        .addEnum(EnumDefinition.builder().value("baz").build())
+                        .addEnum(EnumDefinition.builder().value("foo").build())
+                        .build())
+                .build();
+        Model modelA = Model.assembler().addShape(s1).assemble().unwrap();
+        Model modelB = Model.assembler().addShape(s2).assemble().unwrap();
+        List<ValidationEvent> events = ModelDiff.compare(modelA, modelB);
+
+        assertThat(TestHelper.findEvents(events, "ChangedEnumTrait").size(), equalTo(1));
+        assertThat(TestHelper.findEvents(events, "ChangedEnumTrait").get(0).getSeverity(), equalTo(Severity.ERROR));
+    }
+
+    @Test
+    public void detectsAppendedEnumsAfterRemovedEnums() {
+        StringShape s1 = StringShape.builder()
+                .id("foo.baz#Baz")
+                .addTrait(EnumTrait.builder()
+                        .addEnum(EnumDefinition.builder().value("old1").build())
+                        .addEnum(EnumDefinition.builder().value("old2").build())
+                        .addEnum(EnumDefinition.builder().value("old3").build())
+                        .build())
+                .build();
+        StringShape s2 = StringShape.builder()
+                .id("foo.baz#Baz")
+                .addTrait(EnumTrait.builder()
+                        .addEnum(EnumDefinition.builder().value("old1").build())
+                        .addEnum(EnumDefinition.builder().value("old3").build())
+                        .addEnum(EnumDefinition.builder().value("new1").build())
+                        .build())
+                .build();
+        Model modelA = Model.assembler().addShape(s1).assemble().unwrap();
+        Model modelB = Model.assembler().addShape(s2).assemble().unwrap();
+        List<ValidationEvent> allEvents = ModelDiff.compare(modelA, modelB);
+
+        List<ValidationEvent> changeEvents = TestHelper.findEvents(allEvents, "ChangedEnumTrait");
+        assertThat(changeEvents.size(), equalTo(2));
+
+        ValidationEvent removedEvent = changeEvents.get(0);
+        assertThat(removedEvent.getSeverity(), equalTo(Severity.ERROR));
+        assertThat(removedEvent.getMessage(), stringContainsInOrder("Enum value `old2` was removed"));
+
+        ValidationEvent appendedEvent = changeEvents.get(1);
+        assertThat(appendedEvent.getSeverity(), equalTo(Severity.NOTE));
+        assertThat(appendedEvent.getMessage(), stringContainsInOrder("Enum value `new1` was appended"));
+    }
 }