When I was in college, my then girlfriend texted me that she was watching Jojo Rabbit. She knew it was one of my favorite movies, and said that she loved it too. She was talking about how Scarlett Johannson's shoes almost have their own personality. So I said,
spoiler
yeah, it's devastating when you notice her shoes on the hanging body.
Turns out she had paused the movie to make some dinner, and hadn't seen this part yet. Needless to say, she was PISSED!
I can't remember what it was specifically, but friend basically ruined a major plot point in Witcher 3 for me fully knowing I was a good ways out from discovering it on my own. As a kneejerk reaction and knowing he was about 20 or 30 hours into Fallout 4, I told him who runs the Institute and what relation that individual has to the protagonist.
He was angrier than I was because I had assumed Witcher 3 turned out the way he revealed, but my spoiler absolutely blindsided him. He never ruined anything for me again.
Tbh this is the best outcome. He learned his damn lesson.