diff --git a/c/common/test/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.expected b/c/common/test/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.expected
index c0a8359320..3148fb8c7c 100644
--- a/c/common/test/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.expected
+++ b/c/common/test/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.expected
@@ -1,3 +1,4 @@
 | test.c:9:7:9:12 | ... = ... | Use of an assignment operator's result. |
 | test.c:13:11:13:16 | ... = ... | Use of an assignment operator's result. |
 | test.c:15:8:15:13 | ... = ... | Use of an assignment operator's result. |
+| test.c:17:6:17:13 | ... += ... | Use of an assignment operator's result. |
diff --git a/c/common/test/rules/resultofanassignmentoperatorshouldnotbeused/test.c b/c/common/test/rules/resultofanassignmentoperatorshouldnotbeused/test.c
index db0a45384e..031eee8244 100644
--- a/c/common/test/rules/resultofanassignmentoperatorshouldnotbeused/test.c
+++ b/c/common/test/rules/resultofanassignmentoperatorshouldnotbeused/test.c
@@ -13,4 +13,15 @@ void test() {
   l1 = l3[l2 = 0]; // NON_COMPLIANT
 
   l1 = l2 = 0; // NON_COMPLIANT
+
+  l3[l1 += l2] = l3[l1]; // NON_COMPLIANT
+
+  for (l1 = 0; l1 < 10; l1 += l2) // COMPLIANT
+  {
+  }
+
+  while (l1 < 10) // COMPLIANT
+  {
+    l1 += l2; // COMPLIANT
+  }
 }
diff --git a/change_notes/2026-05-18-fix-fp-misra-c-13-4.md b/change_notes/2026-05-18-fix-fp-misra-c-13-4.md
new file mode 100644
index 0000000000..bf6b7c1313
--- /dev/null
+++ b/change_notes/2026-05-18-fix-fp-misra-c-13-4.md
@@ -0,0 +1,4 @@
+- `RULE-13-4` - `ResultOfAnAssignmentOperatorShouldNotBeUsed`:
+  - Fixed false positives and false negatives.
+- `RULE-8-18-2` - `ResultOfAnAssignmentOperatorShouldNotBeUsed`:
+  - Fixed false positives and false negatives.
diff --git a/cpp/common/src/codingstandards/cpp/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.qll b/cpp/common/src/codingstandards/cpp/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.qll
index 04a106b5c4..0556248f26 100644
--- a/cpp/common/src/codingstandards/cpp/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.qll
+++ b/cpp/common/src/codingstandards/cpp/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.qll
@@ -11,8 +11,9 @@ abstract class ResultOfAnAssignmentOperatorShouldNotBeUsedSharedQuery extends Qu
 
 Query getQuery() { result instanceof ResultOfAnAssignmentOperatorShouldNotBeUsedSharedQuery }
 
-query predicate problems(AssignExpr e, string message) {
+query predicate problems(Assignment e, string message) {
   not isExcluded(e, getQuery()) and
   not exists(ExprStmt s | s.getExpr() = e) and
+  not exists(ForStmt for | for.getUpdate() = e) and
   message = "Use of an assignment operator's result."
 }
diff --git a/cpp/common/test/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.expected b/cpp/common/test/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.expected
index 3f2720dd76..d24fa22114 100644
--- a/cpp/common/test/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.expected
+++ b/cpp/common/test/rules/resultofanassignmentoperatorshouldnotbeused/ResultOfAnAssignmentOperatorShouldNotBeUsed.expected
@@ -1,3 +1,4 @@
 | test.cpp:9:7:9:12 | ... = ... | Use of an assignment operator's result. |
 | test.cpp:13:11:13:16 | ... = ... | Use of an assignment operator's result. |
 | test.cpp:15:8:15:13 | ... = ... | Use of an assignment operator's result. |
+| test.cpp:17:6:17:13 | ... += ... | Use of an assignment operator's result. |
diff --git a/cpp/common/test/rules/resultofanassignmentoperatorshouldnotbeused/test.cpp b/cpp/common/test/rules/resultofanassignmentoperatorshouldnotbeused/test.cpp
index 21fb4c0910..0e04d3a7f1 100644
--- a/cpp/common/test/rules/resultofanassignmentoperatorshouldnotbeused/test.cpp
+++ b/cpp/common/test/rules/resultofanassignmentoperatorshouldnotbeused/test.cpp
@@ -13,4 +13,15 @@ void test() {
   l1 = l3[l2 = 0]; // NON_COMPLIANT
 
   l1 = l2 = 0; // NON_COMPLIANT
-}
\ No newline at end of file
+
+  l3[l1 += l2] = l3[l1]; // NON_COMPLIANT
+
+  for (l1 = 0; l1 < 10; l1 += l2) // COMPLIANT
+  {
+  }
+
+  while (l1 < 10) // COMPLIANT
+  {
+    l1 += l2; // COMPLIANT
+  }
+}