An immature guy I used to work with (he was in his mid-30s but acted like he was 20) used to say rude things or make "funny" jokes at other peoples' expense. If someone tried to say something back (even if it was legitimately joking around), he would get mad and pout for the rest of the day. I witnessed it a few times, but never thought I had anything to worry about.

Anyway, one day a group consisting of this guy, my boss, and me were joking around about something. The guy said something "funny", I think basically calling my boss ugly, and I jokingly called him out on it. He immediately got quiet and went back to his desk. I got an e-mail about an hour later where he told me I needed to be more respectful of him.

I put it out of my mind and tried to just avoid him. A few months later, after we had gotten more friendly again, I came in to work pretty sick. I remember having to blow my nose a few times at my desk. He kept giving me shit about it while trying to be funny. After a while I got fed up and told him to fuck off under my breath (semi-serious, but he was being rude and I wasn't in a good mood).

Same thing as last time. He immediately got quiet. No e-mail this time and our boss wasn't around, but I had a feeling I fucked up by even talking to him.

Sure enough, my scheduled performance review came up about a week later. At the end, after going over sales metrics and things like that, my boss (who was actually friends with the other guy), asked if I really told my co-worker to fuck off. I think he was more surprised and shocked that I would do that (I'm known for being quiet and generally non-confrontational). He could have easily written me up, but in the most professional way he could, he basically told me not to do it again and that he thought my coworker was a big baby.