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.
Continuity mistake: When finding out she killed Bethany, Mona's eye make up is ruined from her crying. However in the next shot she's in, her make-up is totally on point.
Character mistake: When Allison falls down the stairs her fiancé says "EMILY!"
Suggested correction: He shouts "Ali", short for Allison, not Emily.
Suggested correction: He shouts, "Ali" which is short for Allison.
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.
