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!
Since the spoiler tag thing doesn't work for everyone (Eternity doesn't hide spoilers, and apparently many other apps don't), I won't say what the spoiler is.
I gave my brother a big spoiler for Jujutsu Kaisen, because I misremembered the order of events and assumed he had already read that part. I've been extra careful when talking about Jujutsu Kaisen with him ever since.
One of my friends gave me a JJK spoiler recently, about a certain "topless" scene.
That sucks, literally my favourite fight in the entire series so far, I would have been so pissed if someone spoiled that for me before I could read it.