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Roundtrip flights from the U.S. to Dublin (DUB) will typically cost you nearly $1,000 or maybe closer to $500, if you get lucky. But soon, hundreds of Thrifty Traveler Premium members will make their way to Ireland for as low as $135. Yes, that's roundtrip.

Welcome to the wild world of mistake fares the unmistakable holy grail of all flight deals. They've become rarer and rarer over the years but we unearthed one last month, with fares to Dublin from both Minneapolis-St. Paul (MSP) and Chicago-O'Hare (ORD) for under $200 roundtrip. Go figure, a deal this good disappeared in a matter of hours but not before hundreds (if not thousands) of travelers booked a trip to the Emerald Isle this summer or fall.

Weeks later without a peep from the airlines or a whisper about tickets being canceled, it's clear the airline is honoring its error. And it was a big one.

Get an instant text alert for the next mistake fare with Thrifty Traveler Premium!

That astounding deal to Dublin renewed interest in mistake fares and rightfully so! But it also begs the question:How on earth does an airline screw upthisbad?

Let's dive into what makes a mistake fare well, a mistake.

From this Dublin deal to $600 business class flights to Southeast Asia, check out the best mistake fares of all time!

The Basics of Mistake Fares The words mistake fare are like catnip. Every traveler dreams of beating the airlines at their own game, booking flights for pennies on the dollar.

But there's not just one kind of mistake fare. Every deal is different. They all share one thing in common, though: They're well, they're all mistakes.

For one reason or another, airlines occasionally sell tickets at a cheaper price than they intended. If you see a plane ticket that looks too good to be true, it could be a mistake fare.

Sometimes, it's not so clear: Was Virgin Atlantic's $900 roundtrip to London-Heathrow (LHR) in business class a few years ago really an error? Or was it just a reallygood deal?The line between those two has become increasingly blurry lately. Currency conversion errors can lead to mistake fares, especially when the tickets are being sold by a foreign airline. Even in our digital age, fat-finger mistakes on the keyboard happen at airlines. Maybe an employee merely missed a zero on those $600 business class fares to Southeast Asia a few years ago? And in some cases, an airline may leave a critical surcharge or fee off a final ticket, resulting in an accidental-yet-massive discount.

Let's be clear: No matter the reason, mistake fares are incredibly rare. In business class or economy, airlines have gotten better and better at detecting their mistakes quickly – or catching them before they're published. So what was once a steady stream of two or three mistake fares a year has slowed to a trickle.

That makes it all the more important to book ASAP and ask questions later: Mistake fares can disappear in a matter of hours, if not minutes. Worst case scenario, use the 24-hour rule to cancel for a full refund tomorrow if you can't make the trip work.

But among these rarest of rare flight deals, there's a common culprit for why they happen. First, some background.

When you book a flight, you see a round price: $179 for a roundtrip to New York or $899 to London, for example. But that flat price is actually built of a half-dozen or more different components: Government taxes, airport fees, fuel and airline surcharges, and finally, the actual airfare.

Take this $777 roundtrip flight from Minneapolis-St. Paul (MSP) to London-Heathrow (LHR) this fall, for example.

Click to see those Taxes, Fees & Charges or plug the flight into a handy tool like ITA Matrix and you'll see there's much more to it.

The airfare itself is only $190 of the final price tag. But you have to throw in a $340 carrier surcharge the airlines like to call it a fuel surcharge, though that's a bit of a misnomer as well as almost $250 in government taxes and additional fees. In many cases, all these extra components dwarf the actual base fare.

But they're all built into your final ticket price you see from the time you start your search until you're booked. That's also why you always wind up paying some cash costs when redeeming airline miles whether it's $5.60 for a one-way domestic ticket or a punishing tab of $2,000 or more for some business and first class flights to Europe.

I know what you're thinking: What happens if an airline screws up and simply forgets to include some of these extra charges fees?

That's exactly what happened with these jaw-droppingly cheap fares to Dublin we found last month.

How One Goof Led to the Flight Deal of the Year to Dublin I probably don't need to tell you that paying as low as just $135 to get to Ireland and back is crazy. This could easily go down as the flight deal of the year.

Here's what a typical flight from Minneapolis-St. Paul (MSP) to Dublin (DUB) looks like over the busy summer travel season, courtesy of Air France's breakdown of all the bits and pieces that lead up to that final $783 price tag.

But here's what that looked like back when Air France briefly sold those tickets last month for $150 or less roundtrip.

Notice anything missing? Let me give you a not-so-subtle hint

That's right:Air France didn't levy the fuel surcharge. Subtract that $550 from the initial, $783 price.Then take that number and subtract the difference in the ticket price between the two examples above ($101 minus $17 equals $84) and what do you get?

A final total of just $149: exactly what many Thrifty Traveler Premium members paid before the fares eventually dropped a bit further to $135.

These dirt-cheap prices were only available when searching and booking through Air France's own website and that was nearly a month ago, so these fares are long gone now. That's a hallmark of many mistake fares: It comes down to booking through one place the sole airline who screwed up when publishing these fares.

Did Air France really mess up, though? Or did the airline do it on purpose? We may never know for sure.

But we can say two things for sure:

Weeks later, these tickets are good as gold. While federal regulators have (unfortunately) given airlines leeway to cancel mistake fares, it's far too late for Air France to backtrack now. When you're heading to Dublin for a summer trip for less than most flights to Chicago or Las Vegas (LAS), does it really matter whether it was a mistake or just an outstanding deal? Bottom Line Mistake fares can be maddening. One minute you see a too-good-to-be-true price to somewhere halfway across the globe. The next minute, it's gone.

These unbelievable deals have become fewer and further between nowadays, but last month's deal to Dublin provided us all a nice reminder that mistake fares still exist plus some useful insight into what makes them happen.