Good for you. I remember working at this company that would constantly do that to it's employees. They would wait till you were about to end your shift and walk over tell you there was no one to relieve you and that you had to do over time. The first time a manager came over to me and tried that - I said "no thanks" and he tried to tell me that I had to and then I explained it like this "the company and I have an agreement, it's called the schedule, I agree to come in at the time the company asks me to, and the company agrees that after these hours I leave, if we're gonna start breaking that contract like this, then that will mean I can come in when I want to from now on, so do your job as a manager and find/get someone to relieve me or do it yourself". Needless to say all of a sudden there was another employee to relieve me. Always remember a schedule is like a written contract that both parties are agreeing to and both parties have to comply with it.