Big demo. Should we test?
Nah - we'll wing it on the day!
The number of times I've seen this. It seems like everyone thinks that everything should just work every time, without giving it a wee check beforehand.
Got a video file in your keynote presentation? It worked fine on computer A - we'll never need to test that the codecs are in computer B... For example.
Biting the hand that feeds IT
