A good-sized headache if you’re United Airlines, which was lampooned after it came to light that it had resumed using the flight numbers of the two United planes that terrorists took over and crashed in the Sept. 11 attacks.

United quickly backtracked, taking UA93 and UA175 out of service Wednesday after the news emerged. But inviting further mockery, the airline explained that the numbers had been “inadvertently reinstated” into United’s systems as they are merged with those of its newly acquired sister airline, Continental. How’s that integration going, guys?

Surely adding to the discomfort in United’s executive suite is the timing of this fiasco. United’s parent, United Continental Holdings (UAL), issued a press release Wednesday saying it was introducing procedures that would provide “a more consistent travel experience for customers” on both United and Continental.

The flight number flub is certainly consistent with the widely made observation that the airlines have a knack for screwing things up. On the other hand, United did quickly acknowledge its mistake and take the numbers back out of service. Perhaps a bit of forbearance is called for?

Not a chance. Refusing to be outdone on the absurdity scale, the union representing United’s pilots lashed out at the company’s “insensitivity and unconscionable disrespect of these sacred flight numbers.”

Taking the memory of 9/11 lightly would be “reprehensible,” as the pilots say, if United were actually doing that. But it’s not like the airlines or the pilots are any strangers to honest mistakes. Is everything sacred anymore?