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.
Plot hole: Jackie supposedly doesn't have any money, but she seems to have enough to travel halfway around the world, following Simon and Lynette.
Suggested correction: As Jackie was plotting with Simon to kill Linnet the entire time, surely its the case that Simon had provided Jackie the funds to be able to do this so that they could enact their plan?
Suggested correction: It's not like "not having money" necessarily means lacking at that exact moment any cash, including the funds for one trip; it can simply mean your lifestyle is not sustainable and you're gonna run out completely dry or be forced to sell your remaining properties soon if you don't change anything. The novel makes it much more clear because the characters actually address it and express concern for her saying that she probably exhausted the small allowance she had for that crazy stunt. Poirot in fact in the novel suggests them initially to just let her go on until she'll eventually be unable to keep up. Back to just the movie, it's also worth noting that she is following them on that one trip, which brought them in Egypt from Venice through Brindisi, so 'halfway around the world' is relative too, it's not like she's been squandering the money in this endeavour for a long time.
