Revealing mistake: As the major tries to convince the Chief Inspector to leave the island, we see Hastings come down the stairs. However, prior to this we can see Hastings waiting just out of shot for his cue to enter. The edge of his hat is visible for several seconds, and when he's cued he begins walking into the shot. (01:15:27)
The Million Dollar Bond Robbery - S3-E3
Revealing mistake: All the Liberty Bonds loaded in the bag of Mr. Ridgeway have the exact same serial number. (00:20:00)
Revealing mistake: Poirot is sitting reading the paper with the front page visible to the viewer. A breeze ruffles the pages and reveals that the interior pages are blank. (00:14:10)
The Tragedy at Marsdon Manor - S3-E6
Revealing mistake: In the reveal at the end of the episode we see a prolonged close-up of the newspaper "Nairobi Daily Press." Not only the fateful article linked to the murder is written in a font different and larger from the one used in the rest of the first page, but you can also see where the fake part abruptly ends, resuming mid-sentence the article of a real (or realistic) newspaper used as base for the prop, back to the right font and consistent style. (00:50:30)
Revealing mistake: When Japp points at the news on the paper about the mine stocks going up, in the close-up you can see easily that only the title and the enlarged text of the first 3 lines talk about the mine. The rest is sports talk. (00:33:55)
The Adventure of the Western Star - S2-E9
Revealing mistake: Japp is waiting for Van Braks to make his move, and when he is hopping on a taxi for the airport, he begins to tail him. When the car moves, reflected in the door is a modern No Parking cone in bright yellow (incidentally, they also pass by a parked car with a FUU 692 plate that is a bit too recent for the series, being an Austin Big 7, first produced in 1937). (00:41:05)
Revealing mistake: The phone book has a LMS logo on the page edges in the close-up where we see Poirot actually flip through - the logo disappears when he is moving the book in the rest of the scene. (00:16:10)
Revealing mistake: As he scolds Hastings for his pancreatic hullabaloo, Poirot drinks his tisane from an empty cup. (00:02:55)
Revealing mistake: The stuntman smashing through the window is quite obvious - he does not even have the hair styled like Hastings'. (00:31:30)
Revealing mistake: Towards the end of the preliminary murder scene inspection, look at the victim in the last close-up, following the part when Poirot picks up paper from the basket. Specifically, the gun hand: the pinky finger of the supposedly dead person, after being too long in the unnatural position, trembles a little. (00:07:30)
Answer: He definitely says "Belgian", but the subtitles get it wrong and show him saying "American."
Wblank71