Plot hole: When Sarah Paulsen pulls the necklace out of the water, the necklace clasp is obviously closed. Since the circumference of the necklace is too small to fit over the top of Anne Hathaway's head, why would anyone believe that it fell off her when she was running to the bathroom? The only way that could have happened is if the clasp (which we were earlier told can only be opened by a magnet) had opened. The trained security guards wouldn't have suspected something wasn't right about a closed necklace falling off over of her? They were following her. They saw that at no point did she lower her head on her way to the bathroom for the necklace to fall off over her head to begin with. It also could not have fallen off her when she was vomiting into the toilet in the bathroom since the clasp was closed. (01:18:25)
Plot hole: When the necklace is pulled out of the water (closed) and placed back onto Daphne's neck, they couldn't have used the magnet because the replica didn't have that feature, so the security guards should've noticed the fake right away. To put it back on her neck and to take it off again to go back to Cartier would have required the magnet, which would not work on the replica. (01:18:25 - 01:19:19)
Plot hole: The ending sequence is full of holes. 1) How did Nine-Ball get inside without an invite? Sarah Paulson can't work that much magic with the guest list. 2) How did Constance's employers not notice that she was gone - and/or that she was now dolled up with a dress? 3) Same problem for Amhita. 4) Constance, Amhita, and Nine-Ball all have elaborate makeup and hairstyling, the kind that takes hours and several experts to do. Which they don't have.
Plot hole: When the necklace has been smuggled to Amita, she goes for a "bathroom break." She then opens the baby changer and tampon dispenser to reveal a jeweller's toolkit to allow her to break the necklace down. How and when did they get those in and out without anyone noticing? They would have had to make a fake changer and tampon dispenser, somehow replace the real ones with the fakes before the Gala began, and then somehow put it all back to normal before leaving? Not possible.
