Surprise cash counts are to be performed by the branch manager or teller supervisor randomly throughout the quarter. Each teller drawer should be reviewed on different days, but sometimes the manager remembers on the last day and does them all that day. That is not good, but it happens. Also, what everyone else said - last day, leaving on vacation, over/short over a certain $ amount.
Audit Dept surprise audits for the branch happen less frequently, but all tellers are audited on the same day.
You gain education by reading the fine print. You gain experience by not.