The American company, which only launched its service 18 months ago, employs a team of eight around the world who spend their days searching for such fares, before alerting Keyes's customers.
Deals have included return flights between London and Los Angeles for $355, or Glasgow to and from Bangkok for $523.
How do mistake fares happen?
1. An airline means to set a fare at $1189 and accidentally sets it at $189.
2. Variations in foreign currencies, especially following a major currency devaluation, let you book a far cheaper flight.
3. The particular routing of the flight allows certain fees like fuel surcharges to be dropped from the price, when typically they would be included.
4. An error in communication between an online travel agent and an airline causes a mistake fare.
5. Outdated internal technology and pricing systems used by airlines, particularly carriers that have been around for decades, can initiate a mistake fare and make it difficult to fix quickly when they do pop up.
Last week, Air New Zealand advertised fares as low as $499 return from Auckland to Honolulu and $159 one way in an email to customers, but when they clicked through, the fares were corrected to $499 one way and $992 return.
Though Keyes specialises in mistake fares, his experience of seeking cheap flights has taught him more about how the industry works.
For one, he says there is no such thing as the Tuesday 2pm myth - that booking a flight at this time of the week is traditionally cheaper than other days or times.
"This was the case a couple of decades ago when airlines loaded their fares once a week, but hasn't been true for years," he says.
When it comes to finding the cheapest fares, he highlights three common mistakes made by the travelling public.
Firstly, chase the fare not the destinations. Keyes says rather than looking for a location then the flights, why not search for several holiday hotspots, getting an idea of where offers the best-value seats.
"Plus, once you get to a different continent, it's typically really easy to hop on a budget flight to your final destination for under £80," he says.
Secondly, time your booking right.
"Many people think prices will drop at the last minute or think a month is far enough in advance to book peak summer or Christmas flights," says Keyes. "Wrong."
"I typically recommend booking when you see a great fare one to three months in advance for domestic flights and two to eight months for international flights. If you're booking for peak summer, I'd add a month or two onto these recommendations, and if you're travelling during low season it's more likely cheap fares will pop up closer to departure date.
"Prices tend to jump in the last month or two before departure, so you don't want to cut it too close. On the other hand, if you book too early and settle for a not-great fare, you're missing out on potential future sales."
And finally, Keyes says it is worth thinking about where you're flying, and alternative ways of getting there.
"If you were to search a flight comparison website for a flight from London to the Galápagos Islands, it'd typically cost about $2100 return. Yikes," he says.
"A round-trip from London to Quito, though, regularly drops down to the $700s, and a round-trip from Quito to the Galápagos usually costs about $260. So in this case dividing your trip into two separate itineraries (London to Quito, and Quito to the Galapagos) would not only save over $1000 per person, but you could also spend a few days in Quito as well."
This article originally appeared on the Daily Telegraph.
