"Even if the bank doesn't notice its mistake, the money's real owner will. When the bank investigates their customer's complaint and finds the money in your account, they'll naturally question why you didn't report it. If you've spent the money or transferred it to another account, you'll have to pay the bank back and may face criminal charges. That Georgia couple, having spent almost all of the $120,000 before the bank uncovered the mistake, were convicted of theft and ordered to repay more than $100,000 to the bank."
That's from Experian (credit reportin bureau)
