What has happened many times is I argue with my friend or my parent and "win" while they're defending their position to the teeth. Months later, they present my argument to me as their own as if the previous discussion never happened.

I have known one person for whom this was a deliberate policy. He would never (he said) admit to changing his mind about anything. If he did change his mind as a result of an argument, he would merely cease advocating the view he now thought erroneous, and after some suitable lapse of time, advocate what he now believed, as if he had believed it all along.