Continuity mistake: When Poirot and Mr. Ratchett first meet in the restaurant car, Poirot has to correct Ratchett's pronunciation of his name. Poirot then points with his right hand toward Ratchett, but when it cuts to a close-up of Poirot, both his hands are on the table. The angle changes back, and Poirot's hand is once again lifted.
Continuity mistake: During the flashback to the murder, watch closely right after Mary Debenham stabs Mr. Ratchett. One moment she is in the room, the next she is back in the doorway.
Factual error: In the Bosphorus scenes, as the ferry crosses from the Asian to the European side of Istanbul, in the distance large trucks can be seen speeding along the coastal road opposite. Well before their time.
Continuity mistake: Hercule Poirot takes the German maid to her cabin to look for photographs. She removes her suitcase from the shelf and opens it, showing a very obvious shot of a porter's tunic with many very shiny buttons and space for the missing button (evidence found earlier by Lauren Bacall). The shot cuts away and then immediately back, by which time a porter's hat has appeared on top of the tunic covering all of the buttons previously in shot.
Continuity mistake: In one of the flashback scenes, as the drugs start working on Mr. Ratchett, in a close-up he is holding the last anonymous letter in both hands. Ratchett then removes his glasses with his right hand, and places that hand on his forehead. Back to a close-up of the letter and the letter is in both hands, but when it cuts he is holding the letter with only his left hand again.
Factual error: The same locomotive is used for the entire run of the train, from Istanbul to northern Yugoslavia. This is impossible - locomotives would have been changed at the Greek, Bulgarian, and Yugoslav borders, at Belgrade, and perhaps at Nice.
Continuity mistake: While it is fairly obvious that many of the flashback scenes are not the same as the original scenes one in particular stands out: during McQueen's second interrogation he describes Mrs Armstrong as being "gentle and frightened" while in the flashback he says "shallow and frightened".
Continuity mistake: Widmark's eyes are open when his body is discovered. But in the scene he is drugged he is last shown with them closed.
Factual error: The Locomotive pushing the snowplow close to the end of the movie is a French 141 R. They entered service from 1945 on, ten years after the movie is set.
Visible crew/equipment: When Ratchets gets up from the dining room table after Poirot turns him down, you can see the cameraman in the silver buffet on the table.