It's rude to be late for an appointment
Mandy has set up an appointment to discuss extra credit work with her professor, who has told her that he has a doctors appointment right after their meeting time.
Mandy wants to make the meeting with her professor.
Mandy sets an alarm, so she can get to her appointment on time.
Mandy neglects to set an alarm and therefore arrives late to her meeting with the professor.
Mandy and her professor work out a suitable extra credit arrangement, and he makes it to the doctors office on time.
Mandy's professor waits for her to arrive and ends up missing his doctors appointment.