You are out the 50 bucks, and if you try to return it to the bank or take it anywhere else you may be charged with felony counter-fitting. Simply for passing the bill along, or even trying to, or really even just having it in your possession. Shred it, and buy one of those pens they use to check bills. Check all the bills the bank gives you in the future, in front of them.

Seriously, I've heard of at least 2 people around here arrested for the same thing. They also claimed to have gotten the bills from the bank. Once the bills are in your possession, you are on the hook for anything that might happen.

Goodluck.