You always get at least one person who asks a question just to try and make the person on stage look stupid. We had one today at the end of our async web services presentation.
I was along the lines of have you considered technology X? Unfortunately I hadn't and I said so; but the questioner went on to badger us for a few more question even though we admitted it was something we will have to look at. The questioners tone at the time seem a little bit off a the time but when I got out of the presentation I found out that the person asking the question was a spec lead for X. This really anoyed me as I think it is only polite to declare an interest if you are going to ask some questions. It is not as if we can actually see the questioner due to the spot lights on us.
I don't mind being told I am wrong or have missed something; but it was the tone and sneakiness of the questioner that makes me feel it was a spoiler rather than a question meant to help move everybody understanding forwards.
(I do know what technology X does in that case know, and I am not 100% convinced that it is that much better, than what we presented)