You either a) write a new question, b) put that contestant on a different episode, or c) ask him or her to come up with a new specialty subject.
B is fine, but A and C aren't really viable. Based on modern production practices only, I'm guessing each contestant applied to the show with a specialty category (or few) in mind, and the producers had that prepared ahead of their tape date. Then, the contestants and material would be randomly matched the morning of taping. There probably wouldn't be time to write a brand new main game question, especially in the pre-internet days, and you're not supposed to do that once you know who's playing anyway. To the other point, the contestant shouldn't have to switch their expertise because of the show's negligence.
What SHOULD have happened is somebody checking the pool of games ahead of time and flagging that contestant X couldn't play in game Y, and/or the category with the Ferris Bueller question should have had a spare that they subbed in for just such a reason. Perhaps somebody saw it and rolled the dice that said player wouldn't get that main game question (which she ultimately didn't, but it was close).
-Jason