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)
Other mistake: When Helena Bonham Carter removes the necklace from its case, the clasp of the necklace is obviously closed (or at least it should be-otherwise it would slip inside the case itself from the raised mount that it is kept on in the case when the case is being carried) ; she is carrying the top of the necklace (where the clasp is) in her left hand while her right hand carries the bottom of the necklace. Had the clasp been open in the case, her arm and hand motion when she picks it up with her fingers would not have been so smooth. She goes over to Anne Hathaway with the clasp obviously opened (because we see her put the necklace around Hathaway's neck rather than over Hathaway's head) and then she closes it. When she tries to open it a few minutes later the security guard informs her that she cannot open it; that it can only be opened with a special security magnet - which he then proceeds to use to unlock the clasp. In other words, the necklace would have needed to be opened by the security guard to begin with before Helen Bonham Carter could put it around Anne Hathaway's neck. (01:01:05)
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)
Character mistake: Sandra Bullock's German is very good, but not perfect. She says about her husband, "Gestern hat er seine Schuhe in den Gefrierschrank gestellt," ("Yesterday, he put his shoes in the freezer"). Immediately afterwards, she says "seine Schuhe ins Gefrierschrank," ("He put his shoes in the freezer"). The word Gefrierschrank (freezer) in German is masculine and can never be preceded by the accusative case contraction "ins", the contraction for "in das." She just changed the gender of the word Gefrierschrank (freezer) from masculine in the first sentence (correct) to neuter in the second sentence (incorrect). (01:40:05)
Factual error: This film repeats on of the classic mistakes that countless films have repeated since the invention of personal computers: It shows that letters in a password are discovered independent of each other. In reality, this is not possible. The computers themselves don't know the password; they only know a "hash digest" with which they can only determine if the password is 100% correct, or not. (If you need technical details, look up "Cryptographic hash function" on Wikipedia).
Continuity mistake: In the beginning of the movie when Debbie visits Danny's grave she is standing and there is no bench. At the end of the film she goes back to visit Danny's grave and a bench magically appears which Debbie is sitting on and mixing a martini.
Factual error: A shot of Paul Damanian's computer's screen shows mostly a large wallpaper. At the bottom, there is a taskbar that resembles that of Windows 10. Judging from the task view icon, it is Windows 10 version 1709 or earlier. The taskbar is retouched: The Start button is missing, and the Microsoft Store icon is edited to have the Windows logo cropped out. The taskbar is missing the clock, and the Action Center icon appears before that of the Touch Keyboard icon. There are four icons on the desktop, but none have a label. The icons correspond to "This PC", "Contacts", "Documents" and "Pictures" but none are genuine Windows 10 icons. The "This PC" icon is from Windows 7, while the other three icons are edited versions of what's seen in Windows 10. There is a battery icon but this item appears on laptop computers only; this one is a Dell desktop computer. After clicking on the Search icon (Cortana seems to have been disabled) a task manager appears instead. Moreover, the task manager belongs macOS rather than Windows. No wonder the tasks shown in it moments later are running in the context of the "root" user account. Then an app is executed on that computer called "McCallister Security Visual Matrix Controller 22.214.171.124" but the window chrome indicates that it is a Linux app. (00:48:25 - 00:48:55)
Other mistake: Sandra Bullock is walking into a room at The Met Museum where the painting, "Washington Crossing the Delaware, " hangs. When she walks in, there is no picture right next to the left of this giant painting. However, when the security guard notices a crowd gathering at that spot, there now appears the smaller painting of the "Crossing" (which was vandalized by "Banksy"). (00:42:33)
Character mistake: Sandra Bullock complains that there are no signs that label the areas and directions for visitors correctly. She uses the term "Schilderung", which is based on the word "Schild", which means a sign. However, the word "Schilderung" means description of something, e.g. a suspect, a scene or an experience. "Anweisung" or "Anweisungstafeln" or "Anweisungsschilder" or "Anzeichnungsschilder" would all be valid choices, but not "Schilderung "