Do you know what happens when the bank makes an error in you favor? Only one thing – headaches.
If you ever check your balance and notice that you received a mysterious windfall, do yourself a favor and call your bank. Tell them that a mysterious deposit just appeared on your account and that it’s not yours, they need to investigate, and resolve it quickly. The last thing you should do is spend it despite the adage “possession is nine-tenths of the law,” the money isn’t yours. If you spend it and cannot repay it, you are still liable and must pay it back.
How Bank Errors Happen
In our increasingly electronic world, many of our transactions rely on strings of numbers that can be transposed, mis-typed, fat-fingered without people realizing it. ABA bank routing numbers  are nine digits, credit card numbers  are sixteen digits, and bank account numbers vary in length and style. Those are a lot of numbers and a lot of chances for error. While there are fraud detection and various other prevention mechanisms in place, you can’t always prepare for or prevent human error.
What You Should Do
It’s quite simple – call your bank and tell them. There’s really no other response and you should never ever spend it even if it takes months to resolve. In the end, once the bank figures it out, you’ll have to repay the money. If in the interim you want to put it in an online savings account  to earn a little extra interest, that’s acceptable (as long as you can get the funds out quickly), but never ever spend it and don’t invest it (you could lose it).
What if you try to withdraw it and run? The police will try to catch you as they did last year after a New Zealand couple discovered their bank made a $6.1 million  error in their favor. Westpac Bank gave them a NZ$10,000,000 loan instead of a NZ$10,000 loan and they ran off with the money.
Have you ever been the beneficiary of a bank error in your favor?