If it really is a $650 "bank error", they will eventually find it and you will have to give them their money back. You might also want to turn things around and look at in another light. If the bank made $650 error in their own favor, would you expect them to give you the money back, even if you didn't notice it was missing?
Or let's say you are in a store and the clerk leaves $650 on the counter and goes away. You can take it and know they will never catch you. Would you?