It was supposed to be a moment for celebration: United Airlines observing the delivery of its second Boeing 787 Dreamliner with a flight from Seattle to Chicago earlier this month for a select group of employees, while senior officers, including Jeffery A. Smisek, United’s hard-charging chief executive, served Champagne and took lunch orders.

But before the flight took off that morning, a computer glitch in one of the airline’s computer systems delayed 250 flights around the world for two hours.