You should always do what you say you will
Tim promised Susan that he'd drive her to her physics final exam, but it's at the same time as his favorite team's hockey game.
Tim wants to watch the hockey game.
Tim records the hockey game on his DVR and drives Susan to her exam like he said he would.
Tim buys a ticket and goes to the hockey game, leaving Susan to get to her exam on her own.
Susan arrives at her exam on time, passes it, and passes the course.
Susan doesn't make it to her exam on time, gets a zero, and fails the course.