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 tuns 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.