United Airlines stopped using the flight numbers 93 and 175 after its planes with those flight numbers were used in the terrorist attacks on the United States on Sept. 11, 2001.
On Monday, however, in what appeared to be a glitch, a computer allowed the numbers to be assigned to upcoming flights.
The glitch occurred as part of the merger with Continental Airlines. Computers assigned hundreds of United flight numbers to former Continental flights. The mistake was made on Monday night but wasn't caught until Wednesday, and United responded by immediately removing the numbers from its system.
The numbers were assigned to future flights. Neither flight 93 or 175 ever took off.
On 9/11, United Flight 175, which left Boston bound for Los Angeles, was flown by terrorists into the South Tower of the World Trade Center. United Flight 93, bound from Newark, N.J. to San Francisco, went down in a Pennsylvania field after passengers tried to overpower the terrorists flying the jet.
United Airlines said in a prepared statement that it "regret[s] that these flight numbers were inadvertently reinstated in our system. We have already taken steps to remove them and apologize for the error."
The mistake was met with outrage by United pilots and the Association of Flight Attendants.
"United Airlines' 'inadvertent' reinstatement of flight numbers 93 and 175 -- flight numbers that have become synonymous with the terrorist attacks of Sept. 11, 2001 -- demonstrated an absolute and blatant disrespect of the pilots, crew and passengers who lose their lives on that date," said the pilots.
"The union implores the airline to respond quickly out of respect for the Flight attendants, pilots, customer service agents and passengers who lost their lives on those flights," said the Association of Flight Attendants.