There was an instance on one of my leagues either last year or the year before where the anchor bowler of the opposing team threw at a 10 pin with his left hand (normally a righty) on the fill ball. It had no bearing on the score as we'd won pretty handily, but we informed him of the rule and didn't pursue anything past that. It was his first league, it didn't change the outcome, and he missed the pin anyway, heh.
The thing that struck me as funny about the situation was what he said when we brought it up. He told us that the secretary said he could do that. There were no meetings or anything to discuss it and it wasn't in the league rules. We talked to the secretary on the side about it and showed him the rulebook and he said "Oh, that's a rule?" Needless to say I don't bowl on that league anymore. :P
I do have a quick rule question though while I'm here. I bowl in a house where there's maybe 4-5 lefties on the whole league, and in general they're very good bowlers. I would never do this, but someone said that next time they bowled one of the lefties they were going to throw weird shots lefty in practice to "mess with their oil". Does that fall under unfair tactics or anything like that?