United Airlines (NYSE:UAL) apologized Wednesday for briefly restarting use of flight numbers of two planes that crashed after being hijacked by terrorists on Sept. 11, 2001.Spokesman Rahsaan Johnson blamed the reuse of flight numbers 93 and 175 on a "technical error." He said the airline has taken steps to have the numbers removed from its computers. Johnson said the flight numbers went back into use on Monday night. The airline had stopped using the flight numbers shortly after Sept. 11.