I've heard of a case happening... But I think it would require two people to know each other for a while, even from a distance, since it would probably be dull to rape someone who has no emotional connection to their character and thus cannot suffer from it - or enjoy.

Now anything is possible if the players discuss it first ooc to avoid causing terrible trauma but that takes away the element of surprise. I haven't still managed to get any of my characters get raped or rape someone though must admit it has been in mind. Three of my male characters have been fantasizing about taking someone by force, and not all of them are obvious. Or shit, how Could they be obvious if only one of my characters is considered a bad guy? (And not counting Jaina damnit!! XD ) What keeps me from doing is the fear that the player of the victim would take it personally and stop talking to me.