Pretend someone worked at a company and was paid monthly. A month after leaving they are paid their severance, and should not receive any payment in the future.
A month later, they are contacted by the finance department, apologising that they were paid in error, and kindly asked to send the money back.
What would happen if this money was not returned?
The only results I found online state that the company would take it back from any future payments, however in the scenario above this person has left the company and therefore would not have any future payments.
Edit: I decided to do the right (and by the looks of the replies) legal thing and return the money, after checking with the CEO. They admit to a computer problem on their end, and apologised.
I might have misread some comments/answers, so just to clarify - this payment was made a month after my severance pay when I was not expecting any further wage payments from the company.