Proof
Yeah, the right thing to do is give the money back- but could the bank actually prove the mistake?
Unless the servicing of ATMs is well documented (which it probably is) and there was someone to witness the $50's in the $20's slot (police report?) and you trust the person who made the origial mistake to do the inventory- you can't really Prove that the withdrawls were out of the ordinary.
I bet a 'good' lawyer could beat it.