Personally, I would have just lived on. A nice helping to my checkbook, and if they did in fact make an error, then they may find out about it by themselves.
Would that be ethical? Actually, no.
When playing an online RPG, should you find a glitch in the game that gives you a bonus, the right thing to do is report it and have it fixed. The idea is that if you exploit it, you're cheating. And being thanked after proving integrity is a very fulfillng feeling.
Thus, I can truthfully say I am just as flawed in ethical maturity as anyone else. ._.
~EW~