As an new grad who is unemployed at the moment, I could really use the money, but I ended up calling in and leaving them a message telling them about the error. I also mentioned that I wouldn't complain if they were giving away free money for the heck of it. Grr...I think maybe I'm too moral for my own good.
Anyway, would you have done the same? Or just kept the money and then acted surprised if they called?
This is assuming that you won't have future dealings with them (though as of now I'm not sure if that's the case).