American Airlines has been scrambling to make sure the approximate 15,000 flights from Dec. 17 to 31 have pilots. The mistake was due to a “clerical error,” an American Airlines spokesperson told Yahoo Finance, not a glitch, which initial reports said.

According to the Allied Pilots Association, talks between the presidents of both the union and the airline are finally underway to solve the problem.

The error mislabeled important holiday-week flights as low-priority, letting pilots avoid being assigned to the unpleasant shifts, and was initially announced by the union, which did so because the airline had not responded to the union’s communications at that time.

(Though people on Twitter speculated that a Nov. 28 job listing for a “Director, Crew Scheduling” was a result of the snafu the airline said the job posting was unrelated.)

The day after the news broke, the airline released a statement noting that it had managed to secure pilots for all but “only a few hundred” flights in December, all but solving the problem, thanks to “reserve pilots.” “We have not canceled any scheduled flights,” the airline told Yahoo Finance.

In response, the Allied Pilots Association told Yahoo Finance that this was not reflected in the airline’s scheduling app, Trip Trade, which was filled with thousands of unassigned flights.

In an explanation to Yahoo Finance of this discrepancy, the airline said that the reserve pilots were not reflected in Trip Trade.

“Our estimates are based on the exact same data,” said an American Airlines spokesperson. “The difference is that we take into account the reserve coverage that we know we have for the month — that coverage is part of every airline’s forward schedule.”

This is where it gets complicated.

The ‘firemen’ of the airline industry

Sometimes pilots get sick or can’t get somewhere due to weather or a variety of other reasons. Airlines deal with this by having a certain number of pilots on call as “reserve pilots.”

“You’re like the firemen,” Captain Dennis Tajer, the Allied Pilots Association union’s communications chair, told Yahoo Finance. “They’re called to make sure the flight goes out — it’s a normal process.”

Every month the airline determines what percentage of pilots have to be on call, based on weather data and predictive models. For reserve pilots, whether they have to work or not isn’t clear until the day before the flight. This means that the choice of American Airlines’ wording isn’t quite accurate. The flights are not assigned, the airline just assumes they will be, by the larger number of reserve pilots — which are there for the winter weather.