Nothing major. In my country when people are fired they are entitled to recieve some money based on how long they've been an employee. One time I overpaid a dude who was fired by some 5k USD or so (converting from my local currency) which is nothing major but my boss was pissed. Luckly I just called him and asked him nicely to return the extra money and he did without being rude or anything.
Edit: just as a comparison at the same company once one of our (corporate) clients sent us the payment for a service twice by mistake, some ~200k USD that they had to pay 1x we ended up recieving twice. By comparison 5k is nothing.