A round-trip ticket from New York to Houston sold Thursday for $5 on the United Airlines booking system. The sale was one of an unknown number of ticket sold after a computer glitch at one of the nation's largest airlines began to spit out ultra-low fares for about two hours. How low? How about $2.50 for a flight from Washington, D.C., to Austin, Texas? United Airlines officials briefly shut down the online reservation system before announcing that the problem had been fixed by early afternoon.