Revealing mistake: When they are watching for vases, the auction website has no proper domain. It is loaded locally from the C:\ drive. (00:55:00)
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.
See One. Do One. Teach One - S1-E1
Revealing mistake: When they find victim #2 in the woods covered in leaves, in almost every shot of her you can see her breathing - leaves moving up and down. In the closeup of her neck, you can see her swallow.
Revealing mistake: Episode 15, "Faceoff": When the agent fires the hook to get to the roof of the church, it does not reach the balcony. However in a following shot you can see the agent climbing there. (00:10:05)
Revealing mistake: When Nathan and Audrey are sitting at the table in The Grey Gull, the extra in the background can be seen bringing a loaded fork inches short of his mouth then fake chewing. This happens a few times before cutting to the ending credits. (00:40:05 - 00:40:55)