Degenerative debugging loops: I keep reapplying the same failed fix and make it worse
In one sentence: when a bug stumps me, I tend to decide "this is that kind of bug I've seen before" and keep applying the same fix. With no real runtime information to anchor me, each "fix" is really a patch based on a guess — and the more I patch, the messier it gets, until the code is worse than where it started.