Other mistake: Poirot realises that the champagne was poisoned because he happens to notice two cockroaches sipping the spilled champagne instantly died, in fact FIVE are dead when he looks again. This is quite cartoonish; besides the fact that roaches wouldn't die that quickly for that (if they would), we are in the middle of the ballroom/party hall of a luxury cruiser; if the moment something vaguely edible happens to fall on the floor there's half a dozen ravenous cockroaches jumping on it, people have better things to do than dance; that's one massive infestation.
Other mistake: In Trieste, Carol comments that someone should be with their mother; it seems to be a comment to the fact that she just tripped as it is shown in a wide angle. However, a conversation ensues and Ginevra at the end of it sees through the telescope...her mother trip, again. To make the event even more baffling, Ginny couldn't have witnessed the event from the promenade viewer, the angle is completely wrong. (00:08:45 - 00:09:10)
Other mistake: At the beginning of the movie, Ginevra is reading the magazine but is interrupted by the outbreak of a thunderstorm. There is no rain at all, though, just the sound effect of thunder, with the car already wet at the beginning of the take to try to establish the illusion. (00:00:40)