Well, starcraft II, no spoilers ahead:
Anyway, I was playing through the hardest mode of Starcraft II and completely getting every single thing you could do IE: Finish the level losing less than 2 bunkers, etc... Anyway, it was hard and not exactly meant to be your first play through but I don't often replay games so I just wanted to do it that way.
Anyhow, my young brother finished the easy campaign and gave me a HUGE spoiler, I Was raging for fucking days. I am still slightly pissed off every time I remember it. I never did finish the campaign after that, I did become one of the top 20 ranked players of that time but I was just way too mad about it.
Anyway!! I still hold every spoiler over my brother head. Every single time he asks me a detail which could be a catastrophic spoiler (I tend to read the original work a lot) I tell him "If I told you it'd be a humongous spoiler, also I want you to know how much better I am than you because that time with Starcraft..."