The following test failed in the JDK21 CI job set:

jdk/jfr/api/consumer/recordingstream/TestStop.java

Here's a snippet from the log file:

#section:main
----------messages:(7/369)----------
command: main jdk.jfr.api.consumer.recordingstream.TestStop
reason: User specified action: run main/othervm jdk.jfr.api.consumer.recordingstream.TestStop 
started: Wed Mar 22 15:39:39 UTC 2023
Mode: othervm [/othervm specified]
Additional options from @modules: --add-modules jdk.jfr,java.management
finished: Wed Mar 22 15:39:40 UTC 2023
elapsed time (seconds): 0.806
----------configuration:(3/51)----------
Boot Layer
  add modules: jdk.jfr java.management

----------System.out:(58/2318)----------
RecordingStream outer:
[jdk.jfr.api.consumer.recordingstream.TestStop$MarkEvent {
  startTime = 15:39:40.396 (2023-03-22)
  id = "a"
  eventThread = "MainThread" (javaThreadId = 21)
  stackTrace = [
    jdk.jfr.api.consumer.recordingstream.TestStop.testNestedStop() line: 118
    jdk.jfr.api.consumer.recordingstream.TestStop.main(String[]) line: 57
    jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Object, Object[]) line: 103
    java.lang.reflect.Method.invoke(Object, Object[]) line: 578
    com.sun.javatest.regtest.agent.MainWrapper$MainThread.run() line: 125
  ]
}

, jdk.jfr.api.consumer.recordingstream.TestStop$MarkEvent {
  startTime = 15:39:40.408 (2023-03-22)
  id = "b"
  eventThread = "MainThread" (javaThreadId = 21)
  stackTrace = [
    jdk.jfr.api.consumer.recordingstream.TestStop.testNestedStop() line: 127
    jdk.jfr.api.consumer.recordingstream.TestStop.main(String[]) line: 57
    jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Object, Object[]) line: 103
    java.lang.reflect.Method.invoke(Object, Object[]) line: 578
    com.sun.javatest.regtest.agent.MainWrapper$MainThread.run() line: 125
  ]
}

, jdk.jfr.api.consumer.recordingstream.TestStop$MarkEvent {
  startTime = 15:39:40.445 (2023-03-22)
  id = "c"
  eventThread = "MainThread" (javaThreadId = 21)
  stackTrace = [
    jdk.jfr.api.consumer.recordingstream.TestStop.testNestedStop() line: 133
    jdk.jfr.api.consumer.recordingstream.TestStop.main(String[]) line: 57
    jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Object, Object[]) line: 103
    java.lang.reflect.Method.invoke(Object, Object[]) line: 578
    com.sun.javatest.regtest.agent.MainWrapper$MainThread.run() line: 125
  ]
}

]
RecordingStream inner:
[jdk.jfr.api.consumer.recordingstream.TestStop$MarkEvent {
  startTime = 15:39:40.408 (2023-03-22)
  id = "b"
  eventThread = "MainThread" (javaThreadId = 21)
  stackTrace = [
    jdk.jfr.api.consumer.recordingstream.TestStop.testNestedStop() line: 127
    jdk.jfr.api.consumer.recordingstream.TestStop.main(String[]) line: 57
    jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Object, Object[]) line: 103
    java.lang.reflect.Method.invoke(Object, Object[]) line: 578
    com.sun.javatest.regtest.agent.MainWrapper$MainThread.run() line: 125
  ]
}

]
Outer count: 1
Inner count: 1
----------System.err:(12/775)----------
java.lang.AssertionError: Expected outer stream to have 3 events
	at jdk.jfr.api.consumer.recordingstream.TestStop.testNestedStop(TestStop.java:153)
	at jdk.jfr.api.consumer.recordingstream.TestStop.main(TestStop.java:57)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:578)
	at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:125)
	at java.base/java.lang.Thread.run(Thread.java:1623)

JavaTest Message: Test threw exception: java.lang.AssertionError: Expected outer stream to have 3 events
JavaTest Message: shutting down test

STATUS:Failed.`main' threw exception: java.lang.AssertionError: Expected outer stream to have 3 events
----------rerun:(38/8006)*----------

There was a similar bug fixed back in JDK20:

JDK-8298043 jdk/jfr/api/consumer/recordingstream/TestStop.java failed with "Expected outer stream to have 3 events"