I have.... a memory issue. When I was a small child, around five or so, I had a very few books to read. My family didn't do libraries, so what I had was what I had. I was in the habit of reading all of the books on my shelf, in order, before starting again. One day, one of my relatives (I don't actually remember who) asked me what I was reading. It was Nancy Drew #1, The Secret of the Old Clock.

"Haven't you read that already?" The relative demanded. "Why are you reading it again?"

I realized I had done something very wrong. I didn't know why it was wrong, but it obviously was. So I stumbled out something like "Well I don't remember what happened in it!"

That seemed to satisfy the relative. I internalized the message that it was okay to re-read things as long as you didn't remember them, and I set about training myself to not retain what I was reading in order to be a Good Girl and not have my books taken away. Or something.

So... yeah. Unless I'm making a deliberate attempt to fix whatever it is I'm consuming in my mind, or I've read/watched it so many times that I basically have it memorized anyway, it just disappears. Broad themes, characters, and certain scenes stick around, but the minutia is totally gone. There's no way in hell I'd "pass" a Real Geek test even on some of my One True Fandoms, because my brain is literally wired to not hang onto those tiny stupid details.

Except for the Sea Ponies song. I watched the crap out of that TV special, and I can still sing it from memory!