I remember an almost impossible to diagnose heisenbug I had on an embedded device years ago. It was a small embedded system with no MMU and very poor tooling. It took 3 of us stuck in a room working long hours together for 2 weeks to trace it down. I ended up having to port portions of the code base to an easier to use system and luckily the bug ended up being in that subset of the code. The real kicker on this bug is we could never reproduce it with optimizations turned off so it was a constant battle between adding in diagnostics and trying to keep the bad behavior intact. Every little change we would make would shift were the memory corruption was and the reordering from the optimizations made the changes unpredictable at times. Ahhh, shitty memories.

I can't express the joy when the source of the bug ended up not being my fault.

That bug chase in particular made me extremely weary of people who alter systems to fix bugs without understanding exactly why it made the issue go away. It might not have fixed it but just moved it.