How would you handle this?

[GonzoStyle]

It also depends on what kind of boss you have, my current boss I admited it to him, my past employers I woulda never told them. Depends on if you wanna stay, if you like it there, etc also depends on how to approach it. In the end is the money you "owe" worth the trouble? If your boss is reasonable which by the way he tried to blame you, seems like he isn't, then you two could compromise. Sure the mistake was made but look at it from the other side, it's a simple issue of ethics. You didnt know, they didnt know and now you both know. If you were in their place would you expect your money back? In all honesty I would but i'd also consider the 50/50 split since it was a glitch on their side.