One thing that I always stress in my workshops is that there's no excuse for going over time with your presentation. At a typical conference, this will quite obviously inconvenience your audience, since they may want to change rooms to attend the next talk, or take a bathroom break between talks, or grab a coffee. It also prevents the next speaker from setting up in time and may even derail the entire schedule of at least the conference track you're in. And even if you're the only speaker at an event, consider that people in your audience will have made plans for after your talk.
Fortunately, most speakers nowadays seem to have realised this and manage to finish in time; with their talk, that is.