Once in a while I've seen the following in TestZKFailoverControllerStress:
Unfinished stubbing detected here: -> at org.apache.hadoop.ha.TestZKFailoverControllerStress.testRandomHealthAndDisconnects(TestZKFailoverControllerStress.java:118) E.g. thenReturn() may be missing....
This is because we set up the mock answers after starting the ZKFCs. So if the ZKFC calls the mock object while it's in the middle of the setup, this exception occurs.