You might be interested in the Stupid mistakes I repeatedly make thread
Update:: The comment above wasn't intended to necessarily be a replacement for this topic, but as something similar, right before I left for work this morning.
I'd say that my worst habit, that qualifies more as a 'bad habit', as opposed to what I think of in the context of the other thread, is trying to fix things -- I'll be browsing through code that I wrote years ago, and think to myself 'well, that's a pretty stupid way to do things ... I know of much better ways now', and then modify something. Of course, after it's been working error free for years, I'll end up breaking something, all for the sake of making it 'more maintainable', when it didn't need to be maintained (as it was working, before I screwed with it), or to shave off a few milliseconds, when it didn't need that, either.
Of course, this bad habit was indirectly covered by cog in his comment in the Never, never, never thread.