...this might still be of interest. If you wanted to search for them, I'm sure you'd be able to find some of those "I as on Antiques Roadshow and..." type articles that lift the veil a little. The important thing that comes out of any coverage of the Roadshow is that the experts are competing with each other to get on camera just as much as the guests, if not more.
The part you see on camera is actually the second interview/appraisal. When you enter with Auntie's antimacassar or whatever, you first line up for an expert sitting at one of a number of tables divided by subject. There will be two or three furniture experts, for example. This is all off-camera. They will quickly evaluate it, and that's mostly where the "This is a worthless piece of crap"-type of news gets broken to the owners. So it's a controlled environment.
While you are waiting to get your antimacassar appraisal, the appraiser is looking for some sort of hook they can pitch to the producers. Some are obvious. "Your foster father's grandmother claimed she got this from Kit Carson and that's a common enough style of family legend usually hiding something not worth very much yet I can tell it's one of the only verifiable weavings of this style from the nineteenth century and it's a national treasure worth half a million dollars." That will always get to camera, of course. (Actually happened:http://www.pbs.org/wgbh/roadshow/season/6/tucson-az/appraisals/navajo-first-phase-blanket--200101A48/).
That is the exception, obviously. Some of the stories get to air specifically because they aren't such bombshells. I've seen pieces make it specifically because they're fakes, but the producers think it will be good education for the viewers, for example. Another common sort is the, "You've an example of antique collectible X and let's talk about what to look for when a viewer sees one" story.
As to arguments, well, these are PBS viewers, after all. They're not generally pugnacious types. I've seen obvious "But Grandpappy told me he got this from Babe Ruth himself, what do you mean it's a fake?" moments. That is the exception, however, and those folks seem to be by and large disappointed and disheartened, not angry.
The worst incident I know of was back in 2001, and involved the experts rather than the guests. Two Civil War dealers were convicted of wire and mail fraud for cheating people with low appraisals for expensive relics they purchased at those low prices and then flipped for much more. This all happened outside of the show, of course, but they did use a shill to present some items on-camera to build their credibility.
Wow, that was long. Sorry about the infodump, but hopefully it's useful to somebody.