I'd say not remembering until I start seeing the storyline but not for the reason you may think. Usually when this happens it's after my wife has told me "we've already seen this" and I've said I've never seen it. About 10 min in I'll remember it but if rather re-watch it than tell her she was right so I won't say anything. About 30 min in she'll ask if I remember yet and I'll have to admit I do since I'm not gonna lie to her. Yeah it's happened once or twiceWhat's worse? Knowing you've watched a season of a show and can't remember the storyline or not remembering you've watched something until you start seeing the storyline.
...same thing with me and books. I'll pick up something and start reading and realize a few chapters in that "I know this story."
I do the sane with books as well. Wife tells me that by the time I'm 50 I'm gonna forget we're married. My memory is terrible on most things but I can tell you what stump i caught a fish on 15 years ago, what bait i was using, and what the weather was. Sane with where I killed a deer.