I was standing in line behind this guy at the ATM one day. He took his money, but forgot to retrieve the card, and left the ATM.
I went up to the machine, and found that the card was still in place (anybody could have withdrawn a ton of money off of that thing).
I ended the transaction (the machine was asking if I wanted to make another transaction off of that card) pulled the card out, and ran after the guy.
I caught up to him, and handed it over. He was very grateful (I know of a few others who would have been assholes).
I did what I would have hoped somebody else would have done for me, SAVE MY ASS!