Other mistake: The date on the tombstone above Annie says that Judith Myers was born November 10, 1947, and died October 31, 1963. That would make her 15 (not quite 16 yet) at the time of death, yet everyone says that she was 17 years old.
Other mistake: After David Warner kills a man with his long thin sword, he pulls the sword out and puts it back in its scabbard, but there's no blood on the sword or anywhere.
Other mistake: In every case, the pod needs hours to take over the human, but, in Elizabeth's case, she gets changed in less than 5 minutes. (01:24:00)
Other mistake: As the police lift the body of the 'dead' hotel clerk from behind the desk, you can see his eyes look around a bit.
Other mistake: When Poirot questions Maggie Smith, he repeats what she said about Linnet Ridgeway "three days ago." Besides the discrepancy in dates (it was the day before the murder), Poirot at the time was dozing off on the portside deck (mrs. Ottenbourne passes him by going across) so he could have eavesdropped on the conversation in the near area, but not on that particular one which happened on the starboard deck.
Other mistake: During her fight with ex-hubby Michael, Laura asks him in exasperation "will fifty dollars be enough?" then crosses to her purse and pulls out a hundred dollar bill.