Lauren Rosenbaum I didn't see it coming, but their were hints toward it everywhere when I reread it.
This answer contains spoilers… (view spoiler)[personally after weird events started happening earlier in the book (i.e. when Carrie was walking around at night, when Taft or whoever was talking about the house being haunted) I thought that maybe Cady was dead or that there was some similar Sixth Sense-esque twist, but when I read the ending, I was totally unprepared and was basically in disbelief. things clicked into place after I found out that the Liars burned down Clairmont, but I wasn't expecting that twist at all. (hide spoiler)]