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. Cece is also wearing a different dress.
Other mistake: During the scene after the fund raising dance when Lucas offers to take Hanna home, she calls him Ian when she says that she is going home with Emily and Spencer. Confirmed in subtitles.
Continuity mistake: In the beginning of the show, when Emily and Spencer are at the brew, they are seen holding a blue mug filled all the way with whipped cream. Pay attention to Spencer's mug; the amount of whipped cream changes between shots.
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.