I remember a friend in college who discovered a bunch of extra money in her checking account. The bank made an error. She went on a spending spree. Never found out what happened when the bank wanted its money back.
And in those situations, the bank definitely, always wants its money back. When I was in college in Washington, a bank mistakenly put 20s in the 10s dispenser in the ATM machine. My friends and I took out a bunch of money in a way that got it to dispense 10s (which were really 20s). We thought we got over on the bank. We learned a couple weeks later, when there were extra debits to our accounts, that we did not.
That is stealing and I don’t see why people don’t understand that. My husband cashed a check (at a bank we don’t use but the check was from there) then he realized the lady had given him $50 extra. She recounted it and said no it was correct so he went to deposit that cash at our bank and again realized that she had given him $50 too much. He went back to the original bank and had to argue with the woman to get her to take the $50 back. She finally realized her mistake and thanked him. Since we don’t bank at the first bank, I doubt they would have found the mistake.