Rule #38 – Shut down everything but the presentation software.
It's not the most embarrassing thing that can happen to you during a presentation, but suddenly having your instant message program pop up with a note from a friend is certainly in the top ten. And having your gaffe end up online, to be commented on by random folk with blogs (...like me...) is probably enough to push it into the top five embarrassing presentation moments.

My recommendation to anyone giving a presentation is to always ensure that all unnecessary background programs are shut down. If you don't need the 'net for your presentation, disconnect. Remember - whatever can go wrong, will go wrong. Shutting all the extraneous programs limits what can go wrong to just your presentation software, which is more than enough to worry about.