“This past Thursday, a supplier notified us of a non-conformance in some 737 fuselages,” Boeing Commercial Airplanes chief executive Stan Deal said in a statement to employees that was released to reporters.

According to Deal, an employee working at the supplier “flagged to his manager that two holes may not have been drilled exactly to our requirements.”

“While this potential condition is not an immediate flight safety issue and all 737s can continue operating safely, we currently believe we will have to perform rework on about 50 undelivered airplanes,” an issue that “could delay some near-term 737 deliveries.”

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Boeing has struggled over the past year with difficulties on its star plane, the 737 MAX, culminating with a major in-flight safety incident on Alaska Airlines in early January that prompted a sweeping investigation by the regulating body the Federal Aviation Administration.

In his letter, Deal said employees offered suggestions on more than 1,000 improvements since a series of training events following the January 5 incident.

Some have already been taken into account, Deal said, including a process to expedite the purchase of new tools and improve ease of access to certain parts of the plane.