This issue is an follow up of EJBCLIENT-536. The solution was provided and merged but I was struggling to create a unit test for that issue. IIUC in order to reproduce the error many subtle occurrences must happen: cluster discovery, bunch of nodes and their cancellers already added to discovery code cache, adding cancalletion handler from one thread and perfoming cancellation related to the same discovery attempt from another thread. Utimately, I failed to synchronize it all. I'm opening this issue also to bootstrap discussion of feasibility of such nuisanced tests and suggestions for possible alternatives.