The position of the bottle of wine on the table and the amount of wine in the glass changes between shots when John Barratt and Jacques De Gue are talking after they first meet.
When John toasts "to astronomy", he is shown drinking all the cognac in his glass. But, in the next shot, he puts down a glass that is about half full.
The 1950 Delahaye 135 MS Cabriolet belonging to Jean is made in France and has Paris plates but the steering wheel is on the right, indicating an export model for England or other countries that drive on the left.
When John and Jacques first enter the hotel room, a small prop on the side table jumps due to a bad edit.
When Jacques goes to pour John another glass of cognac, there is a noticeable jump cut on the right half of the frame - probably to better line up the pour between the two halves of the picture.
Near the end when John and Jacques are walking around the desk at the foundry, each holding pistols, the film editing done is noticeable by the quick changes in the lighting on the wall behind them.
When the very last line is spoken by Guinness' character, his lips do not move. While it could be argued the line was part of his narration sprinkled throughout the film, the line does appear to be his direct answer to the question posed to him.
During the first conversation between the two principal characters in the Café des Amis the French Count states his personal worth in terms of how many French francs to the English pound (GBP). It seems odd that he assesses his worth in GBP.