The overpressure caused by the SRB ignitions exceeded predictions due to the geometry of the launch pad. This overpressure forced the orbiter's bodyflap away, beyond the design limits of the hydraulic system that controls it. John Young said that if he had known this, he would have ejected, which would have caused the loss of the shuttle.

Ah, I see. But in fact the body flap was not inoperative, and the Shuttle landed safely. So this looks to me like a case where Mission Control turned out to be justified in not telling the crew what had happened.

One thing I wonder about is whether it would have been possible to test the flap while in orbit, to see if the hydraulic lines were actually ruptured or not.