American Airlines is in the midst of revolutionizing its domestic fleet, in good ways and bad. While it has retrofitted more than 600 aircraft with high-speed Wi-Fi, it’s also retrofitting the interiors of some of them with more seats, and less legroom.

Most of AA’s Boeing 737-800s will be modified to add 12 more seats, in order to match the 30-inch pitch and 172-seat capacity of AA’s 737 MAX aircraft. And soon, the airline’s Airbus A321 will start being retrofit to squeeze in 3-9 more seats to “harmonize” the number of seats for the entire A321 fleet at 190. The internal name of this project: Project Oasis.

Now it turns out that American Airlines has hit a snag in this quest for aircraft density. As confirmed by an American Airlines representative, the world’s largest airline has “identified an issue with the quality of work conducted on overhead bins on two of these Boeing 737-800 aircraft” — and as a consequence, decided to ground 14 of the refurbished planes.

After further inspection by American, the work that was conducted on these two aircraft was not up to our standards. Out of an abundance of caution, we have proactively removed from service the additional 12 aircraft that were updated by this vendor and have notified the FAA. We will perform additional inspection work on these 14 aircraft. Though the issue did not impact the safety of flight of these aircraft, we are working with our vendor and the FAA to immediately address this issue.

The grounding of these 14 aircraft has caused the cancellation of 40 flights on Thursday. AA confirms that all affected passengers affected by these cancellations have been rebooked. It’s unclear how long the airline will ground these aircraft and if cancellations will continue into this week.

While the airline would only say that the work was done by a “long-time FAA-licensed vendor,” it’s reported that these 14 aircraft were retrofit by Aviation Technical Services at Paine Field, the home of Boeing.