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.
The initial video was kind of funny, but we could just focus on how fucking awful he is.