A friend of mine was let go from her job. The owners of the company were not very fair with her about severance pay and she had to fight them for what she was due. When they finally did pay up on her severance, somehow, they paid her twice. She promptly put the money in a high interest savings account, locked in for 30 days. She figured it would take them that long to find their error. It was really rather ironic since she was their bookkeeper and they let her go for someone who had the degree she lacked. And that was the person who made the mistake.
They did find their error about a week before the account was unlocked; she sent a cheque back, postdated for that day and kept the interest it earned in that 30 days.