Firts repercussion:
Wasn't an evil campaign per se, but a campaign in White Wolf's 'Wraith' series. For those who don't know, your supposed to play your ghosty character while playing another character's Shadow (something akin to their evil split personality).
NOTE: I'm not insulting anyone here, but I don't know anyone other than my group who played this game
It was a case of a player being too good at evil. While playing, one shadow had kept track of how often he had helped his good half. When said good half asked for more help (I'll call him Mike), the Shadow pulled out his 'little black book', noting the times he helped and wanting compensation. The deal: 24 hours in control; no attempts at stopping the Shadow.
It was in the middle of combat, so Mike didn't have much of a choice.
Now that you have the background...
After combat, the Shadow took his 24 hours. First thing he did: look up Mike's protege! The party had met a small girl (recently dead) that he had taken under his wing. They walked and talked, like nothing was wrong. Then the Shadow convinced the girl to walk into Oblivion, and made sure Mike was watching (major Angst points)!!!
Completely shut down the game! Even though the player playing the Shadow had done what any Shadow would do (tongue twister!), Mike took offense to the action. Game session stopped for the day, and the DM had to write up an adventure to save the girl (she didn't REALLY walk into Oblivion...). Everything smoothed over, but the level of acceptable 'evil' in following games dropped to nil.
Second repurcussion: Having the party rethink their relationship with a fellow Vampire player. He seemed to enjoy his random maiming a women a bit too much.
Third repercussion: Not having played a White Wolf game in a LONG time...