I've not had anything as serious as the previous things in this thread. So here are the more trivial things. Video games and TV. The Phoenix Wright games, for example. I've played them all, and completed them, and loved them. But if I go back and look at them again, I remember the case, and remember what to do and where to go, which ruins it. I with I could un-learn/forget the plot, so I could enjoy going through it for the first time again.
Reminds me of an episode of Red Dwarf, where Holly deletes the memories of Agatha Christie's books, so he can read them again, but after deleting them, he doesn't read them because he's forgotten they exist.
final fantasy 7.... oh how i would LOVE to replay having unlearnt it