For me, probably the biggest spoiler was the reveal that Jacob was a werewof (shape-shifter). I saw the movie first and then immediately read the book quickly thereafter. This was exactly around the time that the New Moon trailer was to premiere at the MTV movie awards. I remember watching a sketch involving Andy Samberg and wolves. I honestly wasn't picking up on it and couldn't see the funny in it. I find it kind of funny, now that I know. But I should've known better, and ended up watching the trailer. And we all know what happens.
So before I could even start New Moon, I already knew that Edward was leaving Bella "forever" and that Jacob was a werewolf. After reading New Moon, I stayed away from any Twilight sites so as not to spoil Eclipse. Thankfully I finished the book unspoiled. For Breaking Dawn, I was spoiled the big one about Bella getting knocked up. But I didn't believe it initially.
The way I it was spoiled because I wa looking at the New Moon trailer on YouTube. While searching for it, there was as fan-made trailer for Breaking Dawn on the page. It had taken a still from the trailer showing a faceless individual pregnant. I immediately cast this off as some sort of dream sequence, because Edward couldn't possibly father a child, lol. Imagine my surprise when it ended turning out to be true.
So anything spoiled in the saga for you as well?