Most likely they just forgot or you slipped through the cracks. It's happened me before and I have two approaches to it. If I haven't worked much then I'll call them and notify them of their error. If I just finished three days of airport standby and the guy that is going to end up doing it because I don't want to hasn't done much then I'll gladly give it to him.

In these cases we aren't screwing the company just the next guy on the seniority list. I wouldn't do it if it's going to cause a commuter to sit S3 on his last day.