When your job calls on your day off and asks you to come in.
Some places are so short handed that their employees are getting called in all of the time. When one works for such an employer, eventually it gets to the point where one cannot relax even on a day off because of the call in threat.
Man, it's nice to have a job and all but I'm always getting called in. I can't sleep properly, plan or do anything because for all I know they're going to call me in at any time.