It happens all the time: A speaker gets on a roll and goes over his time limit, then a sort of domino effect happens, with each subsequent speaker falling further behind until your whole schedule ends up dragging and you either cut a session or two, or find other ways to make up the time, or just let it drag on. But just think about what this is telling your attendees. From Seth Godin:
I doubt this is the message you're trying to get across. Yet another reason to have the hook ready—and use it.