Why does a debugger so often make a concurrency bug disappear?

A concurrency bug is a property of one interleaving out of many. Anything that changes timing changes the distribution of interleavings: a breakpoint suspends threads, a println takes a lock on the stream and performs I/O, and a logging call flushes buffers. Any of them can make the bad interleaving effectively impossible.

There is a memory model dimension too. System.out.println is synchronized, so it inserts a happens- before edge that may be the only reason the values look consistent. Removing a log line and watching a bug appear is a genuine and maddening experience.

What works instead: stress the code with jcstress, run with -XX:+StressLCM -XX:+StressGCM, test on weakly ordered hardware (ARM), use assertions and invariant checks rather than prints, and reason about happens-before rather than about observed behaviour.