@Palpatine:
Well, as the saying goes, sh*t happens. I've been there too, and I know how you feel.
What I now do when I need to demo anything, is to show up at least an hour early and test *everything* before "curtains up". If you don't think stuff is going to work, be prepared to modify or even cancel the demo. A cancelled demo doesn't look too good, but a botched demo looks far worse. A good tip is not to hype the demo ahead of time, that way they might not even notice it got cancelled ;-)
Don't take it too badly, this is a valuable experience that might even save your job in the future :-)
