American Airlines passengers wait in line to reschedule flights at O'Hare Airport on April 16, 2013 in Chicago, Illinois. - 

Computer glitches at home can be frustrating, but what about when they keep an entire airline company from taking off? That’s what happened yesterday, when American Airlines had to cancel over 400 flights and deal with massive delays because employees couldn't access its computerized reservation system, Sabre.

"Airline reservation systems tend to be very, very complex integrated networks," says Ken Colburn, CEO of Data Doctors, which helps companies recover data after a disaster or a system meltdown. "All it takes is one portion of the network to malfunction and it can really cause disruption across the system.

While American Airlines is nearing a merger with US Airways, the two companies haven't yet tried to combine their complicated reservation software and data. Colburn says finding problems within large systems can be like searching for a needle in a haystack. A hardware failure, a wonky piece of code, or that age-old classic: an honest mistake.

"It's entirely possible that it's just one of those really dumb human error things -- somebody tripped over a cord, somebody removed a file," he says. "Most of these systems are really looking for outside hackers or outside issues, and a lot of times it ends up being something really benign internally that just spun out of control."

American Airlines representatives said they would refund cancellations and waive fees for rescheduling.

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