The Federal Aviation Administration said Wednesday that it had recently discovered a new problem with the 737 Max jet that Boeing must correct before the plane is returned to service.

In a flight simulator last week, F.A.A. pilots tested erroneous activations of anti-stall software that pushes down the nose of the Max, two people with knowledge of the matter said. The software, known as MCAS, was involved in two crashes that killed 346 people.

In at least one instance, an F.A.A. pilot was unable to quickly and easily follow Boeing’s emergency procedures to regain control of the plane. The pilot rated that failure as catastrophic, meaning it could lead to the loss of an aircraft midflight, the people said. The situation that was tested is highly unlikely to occur during a typical passenger flight, but the regulator is still requiring Boeing to make a fix, one of the people said.

The F.A.A., in a statement, referred to the issue as “a potential risk that Boeing must mitigate.”

The discovery may erode confidence in Boeing’s assertions, in conversations with regulators, airlines and aviation unions, that well-trained pilots can easily handle a software malfunction based on their understanding of standard emergency procedures.