I once boarded a plane with my wife and as we boarded and looked at our boarding pass to see our seats noticed we had 2 copies of the same boarding pass (the agent printed my wife's twice). I went through Canadian security (where they check your ID and boarding pass 3 times) and boarded without a boarding pass.
Not nearly as good but on an international flight (I think from IST-EWR) someone boarded the plane and tried to take my seat since his boarding pass showed the same seat, turns out he was on the wrong flight (not so crazy, he only needed to get past the gate agent).