When I was a junior high school student, I had a science teacher whose tests — after her brain was damaged in a car accident — always had only true-or-false questions, for which the correct answers were always True.
One of my classmates was a drug-addled bully who had injured a friend of mine. He noticed that I always got all my answers right, and threatened to beat me if I didn't let him copy my answers. He said he would put me in the hospital if I did not comply. I believed him, and let him copy my answers.
That whole school year, though he carefully copied my answers, he never noticed that I always answered T for True. He did notice that I never scored less than 100% on those tests.
Our teacher was too handicapped by brain damage to notice our cheating. She gave pretty good lectures, however, except for a few crazy moments when she told us things like, "God is hydrogen!"
By the time I graduated from high school, the bully who used to copy my answers had stopped beating people, gotten off drugs, and become a good man and solid citizen.
wow... bully aside.. I'm curious, why is the teacher allowed to teach despite her disability?
Well, this was decades ago, she no longer teaches. When she did teach, her disability was not very evident to other adults, because she neither tested nor lectured them. Besides, taking her job away for a disability would have been illegal without a series of administrative hearings that would have been costly for the school system.