I don't think this is the most disappointing appraisal.
The most disappointing I ever saw was the time this woman brought it what she thought was an expensive ivory carving. The appraiser walked her through the whole thing pointing out the various elements and leading her to think it was quite valuable. At the very end, however, he showed her it was just a really good counterfeit and was totally worthless.
I used to watch Roadshow all the time (mostly the American one) and I think I can say the worst appraisal I ever saw was this guy who brought in a whole bunch of Civil War stuff. He had a rifle, a weapons belt, all the stuff a soldier might carry. The guy was so proud and knowledgeable about the stuff. Turned out, everything was fake; it was all modern replicas of very good quality, but easy to spot when you knew what to look for. The stuff was worthless and the guy was heartbroken. That sucked.
Man, that's how Roadshow works, though. You either get the buildup to a sweet payoff by listing all the cool things about an object, or you get the heartbreaking reversal when the appraiser spends three minutes discussing all the neat things about a piece and then ends with ".....or that's what would be interesting about this object if it weren't a Taiwanese knockoff made in 2007. Next!"
This guy is gonna be everyone in 40 years when we all try to dump the box of Beanie Babies or Limited Edition Chex or holofoil special edition X-men comics or whatever dumb shit everyone bought because we all thought it was going to be worth something someday.
Yeah, I've tried to explain to some of my more Ralph-Kramdeny friends over the years that the easiest way to tell if something won't be a collector's item is if it's marketed as a collector's item.