Credit: ABC Family

Spencer has been keeping a secret from the Liars on Pretty Little Liars — one that’s going to shed new light on the night Ali went missing — and it’s going to blow up everything we thought we knew. Not only will Spencer’s relationship with the Liars be compromised, but her relationship with Toby will also take a hit. But is it truly Spencer’s fault for keeping it hidden for so long? New evidence suggests that, similar to Emily, Spencer has a selective memory of the night of the sleep over.

"With Spencer, there are big gaping holes in her memory which will serve as incredibly difficult realizations about what she may have done,” executive producer Oliver Goldstick told E! Online. “There are revelations and realizations that she maybe had confrontations or ugly encounters that she may have blotted out and can't remember.”

So it sounds like whatever happened was traumatic, and we have a hunch Spencer’s “ugly encounter” had something to do with Ali. But what’s going to trigger these suppressed summer  memories? “There are people that are going to remind her, you better believe it! Somebody was taking notes that summer."

The only person we know who has been taking in-depth notes is Ezra. We know that he was seeing Ali that summer (she had lied about her age), but was he also stalking her? Or will Ali’s diary be the gateway to Spencer’s revelation? Whatever it is, we know it’s going to trigger a lot of guilt for Spencer.

“There’s definitely a lot of guilt. Spencer always has a big secret, but this is a big one,” Troian Bellisario told Wetpaint back in January. “And, this is what’s so great about the show. They are unafraid to keep something like this for almost four seasons. We’re going to learn what actually happened the night Ali disappeared, and it’s going to be so fulfilling for fans who have been waiting years for it to all come together. The fans are going to find something out about Spencer this season that the girls didn’t know anything about. It’s going to be a very interesting turn in her life.”

What do you think Spencer has been hiding all these years? Sound off in the comments with your theories! 

Catch the next episode of Pretty Little Liars on Tuesday, February 25, 2014 at 8 p.m. ET on ABC Family.

Crystal Bell is an editor at Wetpaint Entertainment and our resident fangirl for all things Pretty Little Liars. Follow her on Twitter and Google+!