Reminds me of a slashdot thread of "What's your best faking a software demo story?". Apparently when Steve Jobs demoed the iPhone, there was a second iPhone ready to be shown on the huge display in case the one he was holding crashed, and someone in the background was doing the same touches and swipes as Jobs was doing...
IIRC, Steve Jobs had three iPhones at his speaker desk to switch between at moment's notice. I believe he did that once after a wallpaper didn't apply correctly and was instead black.