Revealing mistake: Judging by the interface, the video of Jenna and Toby sleeping together is played through Windows Media Player. When this program is playing a video, the big blue "Play" icon becomes a "Pause" icon. But whenever the video of Jenna and Toby is playing, the "Play" icon is still showing. It's obviously not an actual video being played.
Touched by an 'A'-ngel - S2-E10
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.
Revealing mistake: When the police ring Ashley's phone, we catch a glimpse of the screen as she answers it. The phone is not in call mode.
Through a Glass, Darkly - S5-E14
Revealing mistake: When Jason shows Ali the family photo you can see that Jason and Ali have been badly, almost cartoonily photoshopped in.
To Plea or Not to Plea - S5-E22
Revealing mistake: When Emily calls Varjak's number from the Tony's Pizza receipt, her phone says that it has no service - meaning she should not be capable of making any calls.
The Melody Lingers On - S5-E23
Revealing mistake: When Aria calls Spencer after the first trial scene, Spencer's phone is not in call mode.
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.
Revealing mistake: When Emily gets a phone call from Spencer, the answer button is still visible when she puts the phone to her ear, meaning the phone should actually still be ringing.