If the extra $20 came from an ATM withdrawal, then it's likely they'll never pursue recovering the money for various reasons; it's more of a hassle for many banks to do all the paperwork required to correct an erroneous ATM transaction for so little money.

If the extra funds came from a withdrawal you did in person, you should consider going back to the bank and inquiring if the bank teller that assisted you was $20 short that day. ... if they don't give an answer or say "no", then just forget about it and consider it a bonus for banking there.

Another possibility to consider is that the extra $20 isn't really extra, but perhaps money you already had in your pocket prior to the withdraw.

Ron