A surprise deposit can make a bank balance look much healthier, but money that a company deposits into your account by mistake does not automatically become yours. A payroll department might enter the wrong account number, a business might send a payment twice, or a company could simply make a bookkeeping error.
Whatever caused the deposit, spending the money before confirming what happened can create a much bigger headache than the original surprise was worth. The safest move starts with treating the unexpected cash as a question mark, not a windfall.