Product demo, should it be perfect?

Have you been in a situation where you are looking at a product demo and it failed? Does it sound very common?

Actually it does, majority of product demo in the technology sector fails. Just take in to account the demo that Microsoft did at the Mobile World Congress during Steve Ballmer's key note address. Have you seen that? If not here's a short summary...Steve introduced Windows Phone with awesome capabilities then Joe Balfiore showed the Windows Phone live on stage and then bang!... a few of his demo cases failed. Is that acceptable? Does this outline how bad Windows Phone is?

In my perspective if you demo a product make sure you are well prepared and make sure that your uses cases always work. Or am I alone in thinking as such?