Getting caught badmouthing a co-worker or, even worse, a boss, is the type of faux pas that had the most negative results.
I sent an e-mail to my boss criticizing work someone at
our company had done. The e-mail went to a group that included my boss
... and the person I was criticizing!
I bad-mouthed a senior exec to two co-workers and he walked up behind me and heard every word.
Meant to IM coworker A to talk about co-worker B, but IM'ed coworker B by mistake, and didn't realize until he said "This IS co-worker B!"