Had plenty of spoilers on books, movies and games.
Most twists from the end of A Game of Thrones to the end of A Storm of Swords, for instance - possibly the biggest books-related spoilers.
In games, KOTOR is probably the biggest one. Though you can see easily all the hints, this way.
Nowadays, I'm just glad the few times when I'm not spoiled about what's going to happen - for instance, I was quite glad watching Usual Suspects several years after release that I hadn't any serious clue about the ending.
Zen Bard said:
When I was a wee lad, I was standing in line at the theater (and it was quite a line) to see The Empire Strikes Back. A guy walked out of the previous screening and said "Man, I can't believe Darth Vader is Luke's father!"
We all wanted to kill him.
Plot twist:
That asshole was Homer Simpson
Zhukov said:
Tolstoy's Anna Karenina.
Old classic books are usually published with an introduction in the front, explaining the context of the book, maybe a bit about the author's life, detailing any quirks with the translation, that sort of thing.
Well, the introduction to my copy of Anna Karenina straight up fucking mentioned that...
Happens way too often with classics. Heck, happened to me just last year - forgot which book it was though, but the idiot writing the introduction basically described the whole plot, the twists, who dies and who lives...