As a speaker, there are many things that can go wrong, and potentially derail your presentation. It is not a case of it something goes wrong, but of when it will go wrong.
The difference between a minor incident and a major catastrophe comes down to one simple thing: preparation. Some of the things that can typically happen are:
Computers crash, projectors stop working. Mains power trips. And my favourite, laptop and projector refuse to speak to each other. There are very few presentations that really require PowerPoint, so be able to give your presentation without the equipment if necessary. Arrive early, and give yourself plenty of time to set-up and ensure that all is working fine.
Equipment lost in transit
When traveling to conference venues, you pass through many environments were equipment can mysteriously disappear. Have a backup copy of your presentation with you (in a separate bag to your laptop). Finding a spare machine to borrow is easy; finding another copy of your presentation is far more difficult. Travel with as little equipment as possible (it also makes the traveling easier), and ensure in writing that the conference venue has the necessary equipment.
Microphone batteries die
I insist on new batteries in my microphone (NEW, not freshly charged). Spoiling a $500 presentation for a $2 battery is just plain silly. Keeping a few AA, AAA and 9volt batteries close at hand will quickly rescue you when batteries die.
A little bit of extra preparation will go a long way to ensuring that you give an effective and trouble-free presentation.