Monotony.

I hate bugs that I can't yet figure out how to reproduce, but only occur sporadically in the field. It's this incredibly lengthy cycle of add more debugging, redeploy, wait for an eternity until the bad behavior shows up, analyze the debugging logs, rinse and repeat. It's inevitable that those kinds of bugs come from the absolute stupidest of causes, making you feel like even more of an idiot when its finally tracked down.