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 also explains why the image of the girls through the window doesn't look quite right.
New this month Factual error: While talking on the street, Hanna and Jordan walk into a store called Rosewood Grape Wine after Hanna tells him that she's not going to Allison's without food. However in Pennsylvania, nearly all wine by law is sold in state stores, called Fine Wine and Good Spirits.