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.
Hush Hush Sweet Liars - S6-E20
Revealing mistake: When Aria calls Spencer from the Lost Woods Resort, Spencer's phone is not in Call Mode.