(Associated Press) – Boeing discovered another problem in some of its 737 fuselages that may delay deliveries of about 50 aircraft in the latest quality gaffe to plague the manufacturer.

According to a letter from Stan Deal, CEO of Boeing’s commercial airplanes division, improperly drilled holes were discovered by Spirit AeroSystems, a major supplier that provides Boeing with fuselages.

Both Boeing and Spirit AeroSystems are facing intense scrutiny over the quality of their work after an Alaska Airlines 737 Max 9 was forced to make an emergency landing on Jan. 5 when a panel called a door plug blew out of the side of the plane shortly after takeoff from Portland, Oregon.