From my worst colleague: This man thought things were funny, and they weren't. I bicycle commuted to work, and he thought it funny to buzz me, passing just a few inches away from me, doing about 60mph on his street motorcycle. Another time, I was a bit stressed and needed some downtime so on my way out of the office, I said as much and added "so don't call me unless something is on fire." So, he called me, on my lunch, to tell me the building was on fire. I panicked, thinking all my coworkers lives were in danger, and told him to call 911 and get out of there now. He started laughing.

What did he teach me? He taught me that when I'm sufficiently pissed off, I can be vindictive. I retaliated more than once by doing mostly harmless things like sticking a floppy disk in his PC so it wouldn't boot, and then not helping him fix it so he could do his job (that should date this for ya...). He wasn't computer savvy, and I let him stew in it. That made me ashamed of myself later, and I have never taken the low road again. So he taught me something about myself, and I changed for the better because of it. I still vehemently dislike him and what he did, but that was a long time ago.