Oh, no. That is a total felony. If someone comes to my house and accidentally drops a bank bag with $5,000 in it on my doorstep, it is not my money. I have to turn it in. The same with your bank. If you are paying bills and realize the bank has made an error and increased your balance by a few thousand bucks, that is real money. It belongs to someone. The idea that it is somehow yours to keep is crazy. Just because the bank made an error doesn't mean you get to keep money that isn't yours. This dude is a crook.