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A promo code on Alitalia's Japanese web site over the weekend discounted flights by $315, making some shorter-haul flights free.

Brian Kelly, aka "The Points Guy" was able to book a flight from Rome to Milan - typically $149 - for free with the promotion code.

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"Alitalia has been offering a lot of promotions and discounts recently, so a $315 promo code didn't seem that strange," he said. But Kelly, and all the others who booked flight using the promo code, received emails from the airline stating:

"Thank you for choosing alitalia.com. We regret to inform you that your ticket purchase has not been processed. Any amount debited on your account, will be credited back."

Kelly is an expert mileage programs and is no stranger to mistake or "fat finger" fares. He believes the airline intended the promo code to be for flights originating in Japan only.

Alitalia dis not return ABC News' request for comment.

"People took advantage of a promotion code created and launched by the airline, they [the airline] should honor it. I do think they should be held accountable. Just like consumers are when we book a flight and change our minds, we get penalized, why shouldn't they? "

This is far from the first time an airline has published error fares and then not honored them. Over the summer, United Airlines sold flights from New York City to Hong Kong for four miles and $43 and then cancelled the tickets.

But Kelly said some airlines are better about honoring mistake fares than others. Delta Airlines once had a system-wide glitch that dropped all taxes and fuel surcharges from airfare to Stockholm and Copenhagen. The result was flights under $150. Kelly estimates thousands of people took advantage. Delta honored all the tickets.

And despite not honoring this summer's rock-bottom fares to Hong Kong, United has in the past honored error fares. The airline once dropped a zero from a fare, making business-class fares to Asia $1,000 instead of $10,000. Those people were able to use their tickets.

Even if they're not always honored, mistake fares are worth keeping an eye out for. Additionally, a new-this-year rule from the Department of Transportation may force U.S. carriers to honor error fares. The rule states, ""A purchase occurs when the full amount agreed upon has been paid by the consumer. Therefore, if a consumer purchases a fare and that consumer receives confirmation (such as a confirmation email and/or the purchase appears on their credit card statement or online account summary) of their purchase, then the seller of air transportation cannot increase the price of that air transportation to that consumer, even when the fare is a 'mistake.'"

Kelly suggests following mileage and airfare experts via their blogs and Twitter to get word of these fares quickly. Acting fast is the key to securing the deals. In addition to his own site ThePointsGuy.com, he also recommends TheFlightDeal.com.