As soon as you realize that you have received more money than you asked for, call the bank immediately. Ask for your account balance to confirm the amount remaining in your account as well as the amount recently debited. You must first determine if the teller made a mistake debiting your account or simply when counting your cash.
Deposit the Extra Withdrawn
If you find out that the bank teller withdrew from your account the exact amount that you have in hand, simply go back to the bank to redeposit the overage. This teller simply made a mistake when processing the debit to your account. For instance, it is possible that the teller simply misread the amount you entered on the withdrawal slip.
Do What's Right
The other possibility is that the teller debited your account for the correct amount you requested but simply gave you too much cash back. If you find that the teller gave you too many bills back during an in-person withdrawal, do the right thing. Take the extra cash back to the teller window and explain the situation so that the teller can add it back to her drawer. If the teller's cash drawer comes up short, it could cause her to lose her job.
A best practice is to count your money before you leave the teller window when you make an in-person withdrawal. This helps you to avoid a problem with receiving too little or too much cash. It might hold up the line for a few extra seconds, but this rule of thumb benefits both you and the teller.