Doing something unexpected can be a great way of grabbing your audience’s attention. But don’t try to surprise them by messing around with their schedules. Audience’s expect you to start and finish on time and not meeting these expectations can be a disaster.
Using more time then you’ve been allotted can make people feel that you’re not being considerate of their time. They’ll start to feel anxious while you’re still talking if it becomes clear that you’re going to use more time then you were supposed to because it can make them late to their next appointment. Worse, it’s very difficult if not impossible for an anxious audience to listen and really hear what you have to say. Some people, including me, hate being late more than anything (I blame my father, who always managed to find one more chore to do before we could leave the house). We prompt people are the type who manage to hold long grudges. So don’t do this to your audience, or yourself.
And contrary to the opinion of some, it is possible to finish your talk too early. If you’re scheduled for 90 minutes, don’t wrap up in 60. It can look like you don’t respect peoples’ schedules (they might have used that extra time for something else) or, worse, that you’re not prepared. I attended a conference recently where a speaker finished half an hour early because she hadn’t paid enough attention to see that she’d been given a highly-coveted 90 minute slot. Other speakers would have been glad to have that time to use.