It is rude to expect others will cover for you.
Terry works with Fred as a waiter and is scheduled to work but just learned his sister was in a car accident and is in the hospital.
Terry wants to visit his sister in the hospital.
Terry completes his shift and then visits his sister in the hospital on the way home.
Terry writes an email to Fred and tells him to cover his shift, then goes to the hospital.
Terry is able to buy his sister flowers for her hospital room using the tips he made at work.
Terry is fired when neither he nor Fred show up for his shift that day.