Father Brown and Inspector Valentine walk up a tidal river bank at low tide, yet during their conversation, between shots, the tide comes in completely.
The crest above the chateau gate is nothing like the Flambeau family crest that is such an important part of the plot.
In the library at the top of the ladder Father Brown leafs through a large book of family ancestry. He reveals a page with the symbol of a flaming torch above a coronet. In the overhead shot the torch and coronet sit atop a coat of arms supported by two animals, before text begins with a large illuminated 'D'. But in the dramatic closeup the coat of arms has entirely disappeared. Below the coronet is a single decorative line, and then the text with the same illuminated 'D' begins. In all subsequent shots of the book the coat of arms is still there.
In the stained-glass window behind the (catholic) bishop, there is a portrait of Henry VIII (second from left). Given that Henry was the first king to oppose the pope and separate the Church of England from the catholic church, his face would never be tolerated in this place.
The chateau housekeeper wouldn't be so rude to a visiting Catholic priest.