Sometimes, for no apparent reason, Firefox goes to the previous page. This is often very annoying; for instance, if I was entering information in a web form, I lose all of it. I've been trying to detect a pattern to this but so far nothing.
EDIT1: When this happens, I'm usually not even typing or looking at the screen. I look down at my credit card or whatever, look up and bingo! previous page.
EDIT2: It just happened again! I was typing a comment on Superuser, paused for a few seconds while I pondered something, with none of my fingers anywhere near the backspace key, but holding down the left Shift key. While I stared at the screen, it went back to the previous screen. Maybe a keyboard problem?
EDIT3: And again. Exactly the same situation as last time, except that the cursor was in a superuser answer box, not a comment box. Not that Superuser has anything to do with the problem; it's just where I've been spending time lately!
EDIT4: And again, this time while I was typing in an edit box on my Geeklog blog, but again using Firefox.
EDIT5: It's definitely Firefox, and it's definitely from holding down the left Shift key for an extended period. Bizarre. You might be tempted to tell me not to hold down that Shift key, but I'm not even aware I'm doing it until I go back a page. If it wasn't for this problem, I likely never would have noticed at all.
EDIT6: I've added my observations to Firefox Bugzilla bug report #513218 which seems to be related.
EDIT7: This problem still happens in Firefox 4.