United Airlines caused headaches at major airports in San Francisco, Chicago, and New York this afternoon as computer problems affected its operations both on the ground and on its website.

The airline described the problems as a “network outage.” The extent of the outage extended from United.com, which was down for roughly two hours, to its systems at airports which required the airline to rely on hand-written boarding passes.

“No joke, this is my boarding pass for my flight on @united this evening. Systems are down in STL. #travelerproblems” via twitter.com/jmeg386.

Pictures shared by users on Twitter showed lines wrapped around the inside of Washington Dulles and Chicago-O’Hare airports.

Systems returned to relative normality around 6 p.m., but the three-hour delays mixed with problems related to the Hurricane Isaac re-scheduling will lead to a long night for many United passengers.

United announced via its Twitter account (see below), that “We’ve issued a waiver policy allowing affected customers to cancel or rebook without penalty.”

United’s reservation system outage could hardly have come at a worse time as the airline has been plagued with customer service and performance issues since the merger and integration with Continental Airlines. In fact, the DOT’s Air Travel Consumer Report shows that the airline was the receipient of more than one-third of all consumer complaints against U.S. airlines for the first half of 2012.