I admit that I'm a "genuine idiot" who has never been to a real auction and doesn't know all the conventions. In North by Northwest, Thornhill/Kaplan tries to disrupt an auction in different ways. I understand most of his deliberate mistakes, like bidding down instead of up. However, I don't understand why everyone is outraged when he bids $2000 (video below). What's wrong with this? It's more than the previous bid ($1200) and the increase ($800) seems reasonable for an antique.
I think that if I went to an online auction site like eBay and tried to bid $2000 when the previous bid was $1200, nothing would stop me (though it has been years since I last used that site).