Earlier this year, United Airlines made a mistake. The Danish version of its website offered first-class transatlantic flights for as low as $51.

A view of London, from The Shard. Image by [Duncan] / CC BY 2.0.

 Thousands of people bought tickets, mostly for flights originating in London and flying to the US. The flights were priced in Danish kroner. As an example, flying first-class between London and Honolulu cost approximately 1,275 Danish kroner, or about $193 (ST£114).

In the past, airlines have honored these mistake airfares the same way that a clothing shop that accidentally put the wrong sticker price on a sweater might honor the mistake price.

But this time United canceled the tickets, saying that these fares were mistakes due to a software glitch.

Consumers protested, citing earlier US regulations that shielded them when airlines sold mistake, or so-called “blooper” and “fat finger”, fares.

In May, US regulators changed their mind. An airline can now cancel flights booked on an accidental fare if it can reasonably prove to the US Department of Transportation that the fare was filed by mistake.

So it has become likely that US travelers will never again be able to buy truly expensive flights at mistakenly low prices, such as first-class transatlantic tickets for a song.

In practice, though, things may be a bit different. Airlines will continue to make mistakes, and it may prove to be impractical for them to revoke every single accidentally cheap airfare.

So what is a traveler to do? Trying to catch airline mistakes has become a fool’s game. However, sales happen all the time. It’s important to note that airlines still have sudden drops on airfares that aren’t mistakes and that they will honor. These surprise fares often only last for a brief time, such as for one day, before all the tickets are snatched up.

One website that tracks surprise discount fares — most of which will be honored by the airlines — is The Flight Deal which has focused on flights originating out of major US cities since 2011. A companion site focused on smaller US cities is Fare Deal Alert. Each one has an email newsletter. Deals website Travelzoo also has a daily email roundup of flight sales.

For travelers based outside of the US, it is better to hunt the message board Flyertalk, particularly its “mileage run” forum, for word of sudden discount fares.

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