I have 2 online sins:
Playing Rogue Spear on The Zone, I once joined a "snipers bunker, no crossing" (or rather, "no xing" as people used to say). For those who don't know what I'm talking about, it was a map with bunkers on both sides and everyone played a sniper and you had to kill the people hiding on the other side. No crossing meant that you had to stay on your side of the map. So this one time, I crossed and killed everyone from behind. It took 2 games for the host to ban me =P
Playing Diablo 2 on Battle.net, I was questing in Act 4 with some random people when a guy enters the game. I open the social panel and invite him, but he doesn't accept and wanders in Act 1 for a while and leaves after a minute. Taking the cue, I TP back to town, waypoint to Act 1, then run around the first area for a while and find out that the guy had dropped some unique boots he had intended to mule over to another character. I picked them up and left as he was joining with his other character. I kind of felt bad about this one, but it was a lousy normal difficulty unique anyway.
As far as I remember, these are the only two times I acted like a jerk.
As for offline stuff, I used to read walkthroughs quite often. It's been a while since I used one now though.