Revealing mistake: In the final battle scene, when Edward does a flip around (and then breaks someone's head off), as he flips, you can see that the stunt double doing the flip looks totally different - taller, thinner, lankier, different face and hair.
Other mistake: In the first Twilight, vampires bones break like normal humans, as demonstrated when Alice breaks James's neck. In New Moon, they break like concrete, as the man whose head the Volturi ripped off has a grey stump. In Eclipse, lots of vampires lose bodyparts in the big battle, and they all break like ice, as they have white shiny stumps. In Breaking Dawn Part 2, lots of vampires lose bodyparts in the imagined battle, and they break like china, as they have brown stumps.
Audio problem: When Jacob and Bella are in the car, on the way to Charlie's, Jacob is laughing about "Dracula 1 and Dracula 2" being "creepy." He lets out a huge loud, chuckle in the audio, yet the audio doesn't match his mouth, as his mouth is merely in a tight grin.
Character mistake: In the scene where Carlisle returns with Alistair he asks Edward how many vampires showed up, Edward replies 18. At that point there were only 16. Vladimir and Stefan arrive a few scenes later and are part of the group assembled for battle that totals 18 of the witness vampires.
Plot hole: When Bella drops off Jacob and Renesmee at Charlie's house, she says she has to do an errand and will be back soon. She then proceeds to drive to Seattle, which is about 130 miles east and separated by a large body of water called Puget Sound. Being that I live in Seattle, I know that the most direct route from Forks to Seattle involves driving from the outer Washington coast through Port Angeles, then Sequim, going over the Hood Canal (toll) Bridge, before going to plus waiting for and riding a ferry across Puget Sound from Kingtston to Edmonds, and then at least another 25 minutes driving south on I-5 to Seattle. Even if Bella drove very fast (which she could not possibly do the entire way), it would take close to four hours one way, depending on traffic, wait and ride time for the ferry, stopping to pay at the toll bridge, and general driving conditions. Bella would also spend almost two hours in Seattle conducting her business with Mr. Jenks (which include driving through the busy downtown traffic, parking, going through the mall to the restaurant, and driving through the city again back to the ferry terminal) before returning to Forks. Depending on what time she originally left to when she got back home, it would be late in the evening. There are several other ways to get to Seattle from Forks, but they would take just as long or much longer.