"It really isn't. It's how the industry works," says Ajai Sehgal, Expedia.com's vice president of technology.

Expedia says incorrect prices are displayed on airfare searches about 5% of the time. Other travel sellers decline to say how often fares get switched on consumers, but many experts in airline inventory and ticket-selling say it's happening more often. The Department of Transportation says it has investigated allegations that online booking sites are changing ticket prices based on consumer behavior, but has "found no evidence that this is taking place."

Fare switches can result from several different problems, such as incomplete synching up of different computer systems or travel sellers having stale information, travel sellers and airline executives say. So much searching goes on that travel sellers often don't directly check airline inventory when you are hunting, only when you are buying. Information on the availability of lowest fares is stored—cached—and then used to display fares in searches. Some websites also use sophisticated computer programs that try to estimate which fares are available. When it's time to actually buy, the website then checks with the airline, and may find that an earlier price is no longer available or at a different price.

"It is, in my experience, primarily imperfect data that results in the all-too-frequent 'oops' moments," says Scott Nason, an aviation consultant who ran revenue management at AMR Corp.'s American Airlines.

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