Say, for example, if the professor put the answer up on Blackboard, seemingly by accident, which came in the form of an incorrect answer to a real exam question. They could then give everyone who gives the wrong answer an XF. I'd wager that would be most people, barring the few smartest who noticed the error. Could you really blame people for wanting to get it right and not handicap themselves relative to the other students?
Given that, the university then has a responsibility to make it as hard as possible to cheat. Fail grades for cheating work because they create an incentive to avoid cheating. Except in this circumstance the incentive effect could not work because the student had no idea they'd be able to be caught.