I knew that there was something called "Red Wedding" in A Storm of Swords and that Robb was "briefly" married with Jeyne Westerling (thank you wiki). However, i thought that Jeyne would die and that the Red Wedding was Joffrey's wedding, so it did no harm. I also got spoiled about Arya going blind, about Littlefinger killing Lysa, about Ygritte becoming a lover to Jon Snow (but that was pretty obvious from the beginning), about Arya not being killed by the Hound (after the RW he gave her an axe blow), about Bran and Rickon surviving Theon and about Davos not being killed by Manderly. Sometimes even knowing that someone has a POV is a spoiler, because you know then that the character is not dead. For example, it was a tense reading for me the Jon chapter in A Storm of Swords where he and a few black brothers defend Castle Black against Magnar. Reading this shortly after the RW I was very worried about Jon's life. If I had known that Jon is a major A Dance with Dragons character (and worth many theories about his origins) the fight would have been much less enjoyable. That was the case with Bran, Arya and Davos for me.
I spoiled myself a little, but was too dumb to get the meaning, and I'm glad that I was dumb. I read in the Wiki of Ice and Fire that Robb was "briefly" married with Jeyne, and thought "well, little Jeyne is going to die". When Robb set off for the wedding I thought that Riverrun would be attacked and Jeyne killed. I also heard the words "Red Wedding" and thought they referred to Joffrey's marriage. As for the actual reading, I was so dumb that I only realized that something was going on when "Rains of Castamere" started playing. I was thrilled and shocked. Even more so in the next chapter when Sandor hits Arya's neck with the axe. I thought Arya was dead too, and was astonished. Later I read Jon's battle at the Wall thinking "Jon is gonna die too, please don't let this happen, he is gonna die, there are so many wildlings and so few watchers, Jon wil die too". The tension was even greater than at the Red Wedding itself.