Starting with season 4 episode 21, "She's Come Undone", Spencer will slowly start to realize that maybe she knows more than she thinks she does about the summer Alison disappeared.
Pretty Little Liars executive producer, Oliver Goldstick tells E! Online that Spencer will start to remember more and more - because somebody is going to remind her.
...Brace yourself because Spencer's erratic behavior is only going to get worse and worse. FYI, this is not the first time that little miss Hastings has gone off the deep end. Executive producer Oliver Goldstick explains that when it comes to the summer that Alison disappeared, Spencer's mind was not in the healthiest state. "With Spencer, there are big gaping holes in her memory which will serve as incredibly difficult realizations about what she may have done. There are revelations and realizations that she maybe had confrontations or ugly encounters that she may have blotted out and can't remember," he revealed. "But there are people that are going to remind her, you better believe it! Somebody was taking notes that summer."
Who do you think Oliver is referring to when he says, "somebody was taking notes"?