After a brain surgery, my father developed Anterograde amnesia. Think Memento by Chris Nolan. His reactions to different comments/situations were always identical. If I were to mention a certain word, it would always invoke the same joke. Seeing his wife wearing a certain dress always produces the same witty comment. He was also equally amused by his wittiness every time.

For several months after the surgery he had to be kept on tight watch, and was prone to just do something that was routine pre-op, so we found a joke he finds extremely funny and which he hasn't heard before the surgery, and we would tell it every time we want him to forget where he was going. So, he would laugh for a good while, get completely disoriented, and go back to his sofa.

For a long while, we were unable to convince him that he had a problem, or even that he had the surgery (he would explain the scar away through some fantasy). And even when we manage, it lasts only for a minute or two.. Since then, I've developed several signals I would use if I found myself in an isomorphic situation. I had already read HPMoR by that time, but have discarded Harry's lip-biting as mostly pointless in real life.