My Brother spoiled 6 Sacred Stones For me which really annoyed me but other than that i don't really remember getting a book spoilt - though I do remember the day HP5 came out me and My mate were on the phone for hours flicking through it and discussing what we saw " OMG SIRIUS DIES!" will always stick with me.
My friends claim that I spoilt POTC2 for them which is such a Lie. What happened was - I saw it before them and kept making things up like " Oh yeh Will Turner and Capt. Jack make out" or " I was really shocked when the aliens arrived." Stuff that none of them believed and one of my "Spoilers" was "Yeh at the end Capt. Jack gets eaten by a massive sea monster" which did actually happen, of course they didn't believe me so when they saw it they did get suprised. But now they never know when i'm making it up or what when I "spoil" things for them.