I hesitated over the title of this entry, because there are a lot of dumb career mistakes that you should never, ever make. But there's one that I can think of that is more egregious than most, and many people make it without thinking about it. I don't remember where I learned this one, but it's one of those things that I have learned that has always made my career a little smoother than it probably would have been otherwise:
Never, ever, ever, ever get on the bad side of human resources.
I've seen people do it, and it's almost always just about the worst thing that they can do - their life becomes more and more difficult. This is especially true as the company gets smaller - the person who runs HR almost always has the ear of someone important, and you'd be better off wearing a sign that says "I hate my career" than get in a fight with the head of HR.
It's funny - sometimes the simplest rules are the most profound.