Firefox has had a problem for years where field autocomplete will automatically change capitalization. It makes it impossible for me to log into some of my development tools, because it enforces the capitalization change no matter what. I use Firefox for one tool login and Opera for the other.
Also, I was slamming my head against a desk, trying to find out why my web site caused Firefox to insert usernames into the IRC channel field of a user's profile page. Turns out, Firefox will automatically insert your username into a field that precedes a password field, assuming that if a site is asking for a password, the previous field must be your username. I got really annoyed seeing everyone's IRC channel showing their username instead. Rather than move the password reset field to a different location, I just added a dummy field before the password field, so the extra username is discarded.
Autocomplete is not really a help if it's done badly, and it's often done badly.