Late last week I was demoing a system to our customer. I say system because it combines some very specialised hardware with a Visual Basic app I didn't write and some COM components I did write. Our salesman was present at the demo.
So we're part way through demonstrating the interaction with our database backend when Kevin, bless his soul, chimes in with the suggestion that perhaps the colours chosen to indicate good/bad items should change. I quite agree with him; the original choice of colours was poor. But I could have throttled him on the spot because the customer immediately lost focus on what I was demonstrating and ran down that particular path to nowhere.
It took more than half an hour to put that one to bed and get them back on track. Then Kevin chimed in again. Part of the VB user interface shows a map of physical devices. It's in a fixed size popup window. There's a lot of code behind the scenes to map any arbitrary X/Y layout into that fixed size window and sometimes the items become a trifle small. So Kevin asks if you can resize the window. 'Nope, you can't.' 'Well I think you should be able to!' he says. And once again the customer goes haring off down an irrelevant, for the purpose of the demonstration, path.
I vowed never again to let Kevin attend a demonstration. His points were perfectly valid and totally irrelevant to the purpose of that particular demonstration.