Oh God, I hate that too! I'm sure they've got a rundown of everything they're going to present before their blinkin' shift starts, and probably know exactly how much it's going to go for, surely, if the presenter did genuinely want to question something, they'd do it off air, that way they'd save such dreadful mistakes such as letting a geniune London Treasures bracelet drop down to 3 quid!
....just as irritating is when they don't bother to go through the pretence of checking with the gallery, is when they just blunder in with "This has got to be a mistake...make the most of it" and it's ALWAYS something cheap and crappy and most definitely is NOT a mistake. The day I see a Yarvik, a Satnav, or a telly with plenty of stock going for under £20 is when I'll actually believe there's been a mistake, and in the unlikely event of this happening, it'd be pulled off air as fast as Lola can rip 'er top off!