Most of the time, there's little recourse from pricing mistakes, exactly for the reasons you mention - but there is usually (and definitely in this case - can't

Most of the time, there's little recourse from pricing mistakes, exactly for the reasons you mention - but there is usually (and definitely in this case - can't speak for Mexico) an exception for entering the contract in bad faith. From the article: “The traditional rule at common law,” says Peter Cartwright, a professor of consumer law at Nottingham University, “is that if one party has made a mistake as to the terms of the contract and the other party knows of that mistake, the contract is not binding.” The burden of proof is on the company, and anyone who were in good faith can sue United for breach of contract. But they better have a good explanation for why they suddenly decided to browse for airfares in Danish kroner.