Thomas Fitzsimmons noticed this issue in https://github.com/openjdk/jdk17u-dev/pull/2955

BasicAnnoTests uses @Test annotations for test cases, which a 'posn' representation the expected location of an annotation to check for. Some tests have multiple @Test annotations with the same position:

        @Test(posn=0, annoType = TA.class, expect = "70")
        @Test(posn=0, annoType = TB.class, expect = "71")
        @TA(70) @TB(71) String f;

The tests creates a map indexed by posn, so if multiple annotations have the same posn only one of them is processed: https://github.com/openjdk/jdk/blame/325a2c3f76a45248e6f8baa63477db15e55be705/test/langtools/tools/javac/processing/model/type/BasicAnnoTests.java#L174-L177

The following change should cause the test to fail, but doesn't, because the modified @Test annotation is not being processed due to this bug:

$ diff --git a/test/langtools/tools/javac/processing/model/type/BasicAnnoTests.java b/test/langtools/tools/javac/processing/model/type/BasicAnnoTests.java
index 904e4e78cad..01206faeb6d 100644
--- a/test/langtools/tools/javac/processing/model/type/BasicAnnoTests.java
+++ b/test/langtools/tools/javac/processing/model/type/BasicAnnoTests.java
@@ -697,7 +697,7 @@ class Inner8<@TA(50) T> {
     <T> void m60(@TA(60) @TB(61) String t) { }

     class Inner70<T> {
- @Test(posn=0, annoType = TA.class, expect = "70")
+ @Test(posn=0, annoType = TA.class, expect = "99")
         @Test(posn=0, annoType = TB.class, expect = "71")
         @TA(70) @TB(71) String f;
     }

$ make test TEST="jtreg:test/langtools/tools/javac/processing/model/type/BasicAnnoTests.java"
...
Passed: tools/javac/processing/model/type/BasicAnnoTests.java