A mistake? What one mistake are you referring to?


They failed to clean up the FMS.

PM had a stellar training history and yet...here we are.

IMO-
Voluntary withdrawal is not the same as a failure. Might have been on the path to a failure, but we don't know the reason why.
What we do know is that 2 years later he successfully upgraded to the Airbus, and 5 years after that successfully upgraded to Capt on the Bus.