I was once banned from a CoD4 server for "cheating". "Cheating" on this server basically meant "killing members of the clan running the server". Funny enough, the guy who got me banned (the ban was later lifted) claimed abusing glitches was completely different and acceptable.
Also, I once played on a Counter-Strike: Source server on which it was a bannable offense to shoot through objects to kill someone. That's right- if someone was hiding behind a box, you had to run around the box to shoot him. Needless to say my time spent on that server was measured in the single-digit minutes.