Continuity mistake: On the video of Ian supposedly killing Alison, we see her arm fall to the ground with her "Alison" bracelet visible. But in a later scene where Emily watches Ian put his bag in the athletics room, we cut back to the video and Alison's arm suddenly has a sleeve covering up her bracelet. When we see the video again later in the series, her bracelet is visible again.
Revealing mistake: After the flashback to when the girls first met Cece at The Brew, she looks at them through a window outside when she leaves. If you look closely at Cece's hair, you'll notice a green glow, revealing that she's on a green screen. This explains why the image of the girls through the window doesn't look quite right. Cece is also wearing a different dress.
Revealing mistake: At the end of the episode, Emily receives a phone call from Dr. Sullivan which she does not answer. Look at her phone when she hangs up: Not only does she simply press the "hang up" icon when the arrow indicates she needs to slide it to the left, there is also a magnifying glass symbol at the bottom indicating that it is in fact an image file on the screen and not an actual phone call.