Continuity mistake: The first time we see the scene of Birdie sitting poolside waxing about her and Miles' fame, she brings her left hand up to hold down the side of her large hat, which obscures the camera from seeing Ms. Brand sitting down next to her, and when done speaking she pulls the hat down from the brim. When the scene is shown later a second time from Brand's point of view, when Birdie is talking she isn't holding the side of her hat, and takes it off from the top.
Other mistake: Miles explains the presence of the white bracelets that the Efficient Man slapped on their wrists as biorhythm monitors that act as keys for their room, each assigned to a chakra. Blanc's presence was not expected or announced, but not only Ethan Hawke had an unexplained spare bracelet, but he is also assigned a room, when there are 7 chakras and he brings the guest count to 8. It should also be noted that supposedly they are the "key" to each room, and yet Andi comes and goes from them as it pleases her (ironically the very sort of mistake that birthed the term "Fridge logic") and nobody wears them for the rest of the movie at all because the gag ran its course.
Plot hole: Benoit Blanc ruins on purpose the murder dinner party because, allegedly, someone is really trying to kill Miles, and he wants to warn him that it's not a game. But as we know later, Blanc is actually trying to investigate the murder of Andi Brand and he does not really think that Miles Bron is in danger. In fact he is positively convinced that everyone has motives to kill in order to protect Miles and all he wants to be on the island for is to gather clues and investigate. Cutting the game short does not serve his purpose at all, in fact it defeats it. Had he played along, both Helen and him would have had a perfect excuse to investigate. In fact it is a miracle that nobody went back to their rooms right away after that tense moment, instead of being busy with the opening investigation; you even see that they are groaning and complaining and want to leave. The movie explains away most of the other details but none about this.
Suggested correction: Blanc knew he couldn't hide Helen's identity for long. He said so at 1:15:15. His fear was justified when Duke discovered Helen's identity on the night of her arrival. In short, Blanc and Helen were on a clock.
Regardless of how little time they had left (Blanc estimated "another week", so the leak happens presumably earlier than he anticipated), ruining the dinner does not serve the purpose; gives "Andi" less time to search the rooms, which is what Blanc planned her to do.
If the game was allowed to proceed, Helen would have been forced to play along. Any behavior, other than playing along, would have been suspicious.
Other mistake: When the four 'Disruptors' that stayed buddies solve all the puzzles, they retrieve the invitation from an intricate miniature Glass Onion at the center of the box. When we see the mysterious outcast break the box open, she gets the card from a compartment on the side. While we are not privy to the inner workings of the mechanism, that's obviously not where it was supposed to be. (00:11:00)
Audio problem: Blanc and Lionel are talking about Andi on the ship headed to the island. "In one legal move, he cut her out completely", Lionel says, but the second half of the phrase his chin (as you can tell by his conspicuous beard) is not moving. (00:18:45)
Audio problem: When Lionel tells the others to lift the button at the center of the compass, you can notice that the music box is still playing, but the drum of the machine is still. (00:08:30)
Plot hole: The puzzle box that Miles sends to his friends opens and closes on its own, but its hinges are not connected to any electromotors or kinetic mechanism.
Plot hole: The Distruptors show up at Andi's house right after the killer (Duke literally crosses paths with him) and are enraged and desperate because they can't find her. At the same time, Andi is dying from apparent suicide breathing the fumes of her car. It'd be pretty extraordinary if they were snooping around her property banging at her door and being dumbfounded for, as Claire said, one full hour, but didn't hear a car running in the garage of her small house. (01:32:25)
Plot hole: There is a pivotal moment when Duke poolside casually says; "remember that night when..." That is a mighty strange way for him to mention the very last time the two met, barely two weeks earlier. Miles was not even supposed to be in the country, but Duke, who later jumps straight to casual blackmail once he gets the Google alert about Andi, never mentions that to Miles before or since. Miles cut him off pretending he meant "Anderson Cooper", but for the rest of the movie he struggles and has no leverage on Miles to the point that he pimps his girlfriend out to him. Furthermore, he mentioned the "pancaking" to the other friends, but somehow never mentioned it was Miles. If he was not going to take advantage of that, then he had no reason at all to keep the secret, especially with the lot of them hanging out for an hour waiting at Andi's door. Anyone would have mentioned the fact already back then, or if they realised it was important, would have already started to use it.
Suggested correction: When Duke says "remember that night," he doesn't know that Andi is dead. At this point, he thinks Miles went to Andi's house (just as they all did) and didn't go in. Miles has reason to cut him short, though. Attempted murder is also a crime.
I can't stress enough how absurd it is that he'd tell his friends and co-conspirators that he nearly had an accident without mentioning WHO with. They are banging at the door of Andi without a clue for ages, over a matter that has Miles at the center of everything. That's not how human interactions work. They all are outside her house, and he does not mention the identity of the person responsible for the accident? Again, one could only withhold information like this if there is a purpose.
Feel free to file this one under another mistake entry.
Plot hole: In the flashback, Whiskey fired the speargun at Helen at the bungalow away from the main house as she was fleeing at the beginning of the blackout. She has no light source with her, but somehow in the earlier showing of the events she was one of the first people to pop up at the party room seconds after the blackout started, and with a loaded speargun. (01:06:00)
Continuity mistake: When towards the end of the movie Miles from the top of the stairs shouts "What? You think you're an alligator?" Birdie and Whiskey are in a different position in their close-up. (02:08:30)
Continuity mistake: When Helen enters by mistake Derol's room, there's no smoke visible; she leaves, and in a tighter shot on him a blunt leaves a very conspicuous trail of smoke. (01:41:00)
Continuity mistake: When Lionel says "Why the hell not?" at the unspoken understanding of starting to shatter statuettes, Claire next to him is standing clawing at her own fingers, while in the background a moment earlier her hands were higher and distant. (02:02:30)
Continuity mistake: When Blanc "offers some courage" to Helen, his hand in close-up is around the side of the glass, and it is above it at the cut. She accepts the glass, and her hand changes position on it at the next cut as well. (02:01:05)
Continuity mistake: When Helen shows in close-up Andi's napkin for the first time, look at the position of her finger, next to "Teaching." After a brief cutaway, it changes position. (01:57:05)
Continuity mistake: When Helen grabs the bottle of hot sauce from Blanc's hand, the label turned between shots and it begins in Blanc's right hand when he was just switching it to the left in the previous shot. (01:45:15)
Continuity mistake: When Blanc crouches by Helen telling her that "This is our cover", she is holding the diary with both hands, but in the previous shot her left hand was on the ground, and switches again back and forth. (01:45:10)
Plot hole: Helen looks at her phone for the first time at 9:58. From there, she quickly browses the messages everyone has been sending for several minutes to her, and does a google search. Since she gets interrupted by Whiskey and then this interruption is further interrupted by the hourly 'Dong', she must have ransacked Duke's apartment in barely a minute, which is impossibly fast. (01:41:20)
Continuity mistake: When at the restaurant Blanc says the anti-prophetic sentence "Miles Bron is not an idiot", his arms go from folded to unfolded and vice versa between shots. (01:19:25)
Continuity mistake: In the reveal of the crime that hasn't been committed yet, Blanc points at the table to show the trajectory of the dart. Claire suddenly stands from her seat, but she is back sitting properly a moment later. (00:48:05)
