My multi instructor's biggest pet peeve was wasting time on the radio. If you said "with you" he'd fail an engine. I did it on my first flight checking in with dep and the engine was feathered by 500'. Some time later, while in the heat of battle during a SE sort-of partial panel NDB approach (the DG was failed, still had the Att), I said "with you" checking in with the tower. Yep, boom there went the other one, at least for a few moments to let it sink in..
Everyone has their pet peeves, but he was also my boss as a CFI.