I was on a delayed flight the other day that was then delayed an extra hour while they searched for and removed luggage of people who weren't on it. Nobody got onto the plane after it was announced that this was happening. So, someone got left behind. Or perhaps they had made their own way to Europe. (There was a FlyerTalk thread where someone was being advised to do this.)

PS: It appears that banging on windows and asking a pilot to 'Please come back' doesn't work: https://www.youtube.com/watch?v=Bk_G1Df0_qg