One of the worst examples we've come across was a guy in a star wars RP. The RP was limited to Jedi/Sith, and the guy apparently had a Mandalorian Sith who walked around in Mandalorian Armor. Suffice to say, the character wasn't permitted to post IC.
Even before that, there was a rule in place "No mandalorians" or "No bounty-hunters". Can't remember which one ('twas old guild and updated many times afterwards, so any copy of that is now lost)

Another example from the same RP was sometime after the RP had filled its quota of Sith. One of the Jedi players turned to the dark side simply by being in the proximity of some Sith. And then he actually expected the Sith to trust him.