I would give it back.

BTW; stores do check their till against the purchases made. If you think they make a mistake in their favor, they should at least take your name and number in case they 'find' your money. Everywhere I worked while putting myself through college closed down the register immediately to count down the drawer if a customer complained of being short-changed. Of course, several mistakes might cancel each other out, but they should at least check. If you know what register you paid at, that would help, too.