Pretty Little Liars fans are smart and very attentive, which is why the clue you missed about A.D on Pretty Little Liars isn't on the show. If it had been, we'd have already read about it on Reddit or Tumblr or via any one of the obsessive groups on Facebook. Instead, you can find it on Twitter, which is where Bustle Associate Entertainment Editor Jamie Primeau drew attention to it.
I'd also previously stumbled over that one weird line in the Twitter profile of the actor who plays Spencer, Troian Bellisario. That repetition of "sometimes" stuck out like a sore thumb, but, in the context of the rest of what sounded like a half-funny, half self-serious profile, I'd assumed she was stressing that she was a skilled actor who does other, more hifalutin roles than "this chick... on Pretty Little Liars." So that's on me, because why would anyone play down their role on cable channel Freeform's most popular show?
As such, despite fully subscribing to the Twincer theory, I spaced on the fact that, when the actor stressed that she only sometimes played Spencer, she could have been talking about playing the brunette's British twin as well. It would certainly fit.
After all, Bellisario first find out she was A.D. at the beginning of Season 5, and, from that point on, she and showrunner I. Marlene King continued to work on the character together. She said in an interview with Elle:
This feels consistent with the profile, because what's more hiding in plain sight than advertising on your Twitter that you might not always play Spencer on Pretty Little Liars, you just do that sometimes? It matches with something else the 31 year old told Elle, when she described how "we had to walk that razor-thin edge between keeping the secret and subtly letting the audience know."
This could all be a beautiful coincidence, of course. But given how long the Los Angeles native has been in on the secret and her delight in treading that thin line between revealing and hiding her secret, I'm calling it: this was the one clue that very few fans would have picked up on.