Give it back, of course. If it's a bank error, they will discover it eventually anyway - and who needs the hassle THAT would involve!
I once found a wallet containing $300 in cash. It had been left in a grocery store cart and was just sitting outside in the parking lot. (I later learned it had been there for several hours.)
I took the wallet with me and tracked down its owner by calling several of the telephone numbers I found inside.
Could I have kept the money? Absolutely, it was left alone for so long that anyone could have taken it and I wouldn't have been suspect. I could have used it, too; my daughter was in the hospital that week and I was going through some touchy financial times.
Did I take it? Nope. Luckily for me, I still have a conscience.
P.S. I did keep the $4 I found on the sidewalk a few days ago, though.
