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)
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)