Plot hole: The spell Magicae Maxima works, but it shouldn't have worked at all because there was one ingredient missing, the petrified spider.
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.
Plot hole: On Christmas morning, Karen went to the grocery store (open on Christmas!) where she donated $5,000 via credit card to the "Homeless Mother Shelter." Hours later, the $5,000 already showed up on Chris' laptop as deposited into the shelter's account. It takes days, not hours, for credit card transactions to be processed - especially if the transaction occurs on a holiday. (01:14:10 - 01:23:20)
Plot hole: When Cage and his wife and mates are going to the FBI safehouse the FBI agent shoots 2 baddies two separate times whilst Cage etc. are ascending the stairs, yet they don't hear the gunshots.