Several years ago, I was going through a huge amount of stress and numbing out to tv to escape. Now and then I'll start a new series and realize that I've already seen it - but it was always during that super stressful time when I guess my brain wasn't making memories properly.

Only when I've had a drinking problem.

I used to be high a lot. A LOT. And yeah, I've completely forgotten having seen shows/movies, only to restart and realize "Hey, I've seen this..."

Don't do drugs.

It has happened to me Sometimes I think it’s because I fell asleep watching something that ended, and then the next thing started playing automatically. Eventually my tv will ask me to confirm if I’m still watching and if i don’t it will just stay in a paused state waiting for me to answer.

Episodes of like a sitcom or something yes, but not a whole series.

What I mean is to start watching a show that you think is new to you, that you have no memory of previously watching, only to find that the network (via the progress bar) shows that you've seen.

This has happened to me a few times, enough that I make sure to delete shows I've seen from my watchlist. Make me feel better and tell me you've done this too.