Continuity mistake: In Joanna's bridal shower she gives a necklace to Marni. When Marni is talking to her nanny the necklace disappear between shots.
Continuity mistake: In the post-rehearsal dinner scene where Marni and Joanna start fighting and throwing dishes, Joanna dumps a bowl of green soup over Marni's head. As Will is chastising both of them, the green slime on Marni's clothes and hair changes in each shot when the camera cuts back to Marni. It's most noticeable on her bangs, which are sometimes plastered flat against her forehead and other times are separated into spiky pieces.
Continuity mistake: Marni has acne at the rehearsal dinner, but then when she's sleeping, which is presumably a few hours later, she has no acne.
Suggested correction: It wasn't acne. It was ant bites from when she fell on the run.
Factual error: When Jamie Lee Curtis drops the earring down the drain, she panics and kicks the pea trap off the drain. This causes water to spew from the pipes which is possible if she accidentally kicked the water pipe but when she replaces the pea trap by hand, the water cuts off which would not happen, as the drain has nothing to do with running water.
Continuity mistake: Marni's mom puts the blindfold over his eyes, then there's a shot where you can see just a small side profile of his mouth/chin/neck and the blindfold is around his neck. Then in the next shot, it's back over his eyes again.
