In an earlier poolside scene as Birdie Jay and Peg discuss Miles Bron, Birdie Jay is clearly holding her hat as a crucial action takes place behind her. When the audience later revisits the scene from Helen's perspective, Birdie Jay is fiddling with her sunglasses as she says the same line, and does not touch her hat until she eventually removes it from her head.
When Benoit Blanc discovers the fax machine, his suit is perfectly dry, though he was in the pool the moment before.
When Benoit asks Miles to open the locket, Benoit is standing, and Duke is sitting next Benoit with his face turned towards Miles. In the next shot, Benoit is sitting, and Duke turns his head from facing Benoit to facing Miles.
When Claire gets up to go after Andi, she puts aside her things, with her laptop lying flat and the bag on top of it. A moment later her empty seat is shown with the laptop lying askew on top of the bag.
After Miles and Blanc leave the dinner table after the staged Miles killing, the guests are all lounging around having after-dinner drinks. However, when they showed the dinner table the dinnerware setup hasn't been touched. It looks the same as when they first sat down at the table.
Benoit Blanc requests that, after Duke's death, Miles call his boat and asks them to come immediately. Shortly after, Lionel comes back to tell Blanc that the boat can't come until after 6am as the Banksy dock was set to low-tide height. There is no significant tide in the Greek Mediterranean that would result in this type of problem.
The real Mona Lisa painting is only 2' 6" x 1' 9", a lot smaller than the one shown in the film.
When the friends are solving the puzzles that open the invitation box, Claire says the chess game puzzle is "a chess endgame." All the pieces are on the board, so it is actually an opening.
While arriving on the beach, Miles is playing the song "Blackbird" by The Beatles and refers to the guitar he's playing, a Martin D-28, as "the guitar he wrote it on" (referring to Paul McCartney). This could be regarded as a goof as Paul McCartney is a left handed player and the guitar Miles is playing is a right handed model. However, when the song was recorded in 1968, Paul McCartney played a right handed Martin D-28 that had been restrung upside down for a left hander to play. It is therefore entirely conceivable that Miles bought the actual guitar and had it restrung back for a right handed player.
Whatever the Efficient Man sprays into the mouth of the guests, which is supposed to give them COVID immunity, would have been either globally everywhere by the point, or illegal to distribute as a medicine for COVID everywhere in the world (with many real world examples). Miles Bron could have been destroyed without Benoit Blanc ever boarding the ship, but nobody thinks about this later.