I guess I was wondering if such things happened, off camera.
I mean, obviously they're not going to televise some poor schmo standing up and yelling "you lying bastard!" to the presenter, and then storming off. Or breaking into tears and trying to attack the presenter, or something.
One assumes that in this world of connected media, even if such a happening happened but didn't get televised, we'd still hear about it.
"Upset owner bashes Antiques Roadshow presenter with $20 million painting that turned out to be worthless reproduction!"