Very rarely vmTestbase/nsk/jdi/stress/serial/mixed002/TestDescription.java is failing:

Execute debugger: nsk.jdi.ObjectReference.referringObjects.referringObjects003.referringObjects003@3d6d6825
debugee.stderr> Debuggee: received the command: COMMAND_EXECUTE_DEBUGGEE:nsk.jdi.ObjectReference.referringObjects.referringObjects003.referringObjects003a
debugee.stderr> Debuggee nsk.jdi.ObjectReference.referringObjects.referringObjects003.referringObjects003a : sending the command: ready
debugee.stderr> Debuggee: received the command: startThreads:15
debugee.stderr> Debuggee nsk.jdi.ObjectReference.referringObjects.referringObjects003.referringObjects003a : sending the command: ready
debugee.stderr> Debuggee: received the command: stopThreads
debugee.stderr> Debuggee nsk.jdi.ObjectReference.referringObjects.referringObjects003.referringObjects003a : sending the command: ready
# ERROR: Unexpected size of referenceType.instances(java.lang.ThreadGroup): 3, expected: 4
The following stacktrace is for failure analysis.
nsk.share.TestFailure: Unexpected size of referenceType.instances(java.lang.ThreadGroup): 3, expected: 4
at nsk.share.Log.logExceptionForFailureAnalysis(Log.java:432)
at nsk.share.Log.complain(Log.java:403)
at nsk.share.jdi.HeapwalkingDebugger.checkDebugeeAnswer_instances(HeapwalkingDebugger.java:116)
at nsk.jdi.ObjectReference.referringObjects.referringObjects003.referringObjects003.doTest(referringObjects003.java:181)
at nsk.share.jdi.SerialExecutionDebugger.executeTests(SerialExecutionDebugger.java:294)
at nsk.share.jdi.SerialExecutionDebugger.doTest(SerialExecutionDebugger.java:233)
at nsk.share.jdi.TestDebuggerType2.runIt(TestDebuggerType2.java:214)
at nsk.share.jdi.SerialExecutionDebugger.main(SerialExecutionDebugger.java:65)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
at java.base/java.lang.reflect.Method.invoke(Method.java:578)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:127)
at java.base/java.lang.Thread.run(Thread.java:1596)

So far this has only happened once during CI runs, and I just saw it in my ad-hoc test run.

Note there is a related LOOM issue JDK-8262488, but that does not appear to be the same problem since it was very reproducible with a number of tests, and seems to have gone away. Also, it only happened with running with the debuggee virtual thread wrapper, and the failures in this issue are without.