The vast majority of bank tellers are highly honest people, dedicated to serving the public and protecting the bank's assets. In the very rare cases when an individual with less than honorable intentions manages to slip through the cracks and get hired, there are systems in place to protect the bank's assets. Each bank's policies will vary, and most will have their policies available to their customers either online or in the branch, so if you have questions about your particular bank's policy, ask for that information.
Once a teller runs a transaction through her computer system and it prints out a receipt, the transaction is credited or debited from the client's account. Provided the teller gives the client a receipt for a cash deposit, the transaction is final and it is assumed the teller has put the cash in their drawer. Teller cash drawers and reserve cash vaults are counted daily to verify the amount the teller holds. In the evening, the counts are compared with the computer's record of transactions for the day. If the total is out of balance the drawer is recounted by a supervisor. Typically, the amount of outage is recorded and a paperwork audit performed.
Teller differences not resolved in the branch may be sent on to an internal auditing department which examines teller transactions for errors or audited by the bank management. The difference analyst or manager reviews the teller's transaction tapes and paperwork to see if a transaction was incorrectly registered in the computer. If the difference is not found in the paperwork, the branch is notified and a note is made on the teller's employee record.
Typically tellers are allowed a certain amount of overages per calendar year before the teller is fired or investigated for theft. If the amount lost is below that amount, a running tally is kept and the teller is on warning until the end of the year (when their balance resets to $0) or the teller goes above their set limit and is fired.