Depends how soon they notice they've done it. I think the laws give them a certain time to notice. If they've noticed already, or notice soon, you'll be expected to give them the money back or it's some type of fraudulent shit. Otherwise, if they don't notice after like a year, and you can say you thought it was like over-tax repayment or something. Then you'll prolly be alright.

I wouldn't risk it.