"Listen very carefully. I shall (will) say this only once" was never meant to be a catchphrase. It was said once and it got a laugh, so the character of Michelle said it in every subsequent episode. Many of the show's famous catchphrases were created by accident.
David Croft said the inspiration for Officer Crabtree, the policeman (played by Arthur Bostrom) who speaks spectacularly bad French, came from Edward Heath, former British Prime Minister, who spoke French with a broad English accent. In real life, Arthur Bostrom speaks French fluently.
When Edith catches René embracing another woman and asks for an explanation, one writer sets up the situation, while the other writer writes the excuse René gives.
The original painting of the Fallen Madonna with the Big Boobies, a major plot point of the entire show's run, was given to production designer Shaun Moore after the series ended in 1992. In April 2007, he auctioned it for £4,000 to raise money for the charity the Ace Centre, which helps children with communication problems in Oxford. In late 2018 East Bristol Auction House resold it for £15,000 - genuinely to a buyer in Nouvion, France. It bears a small tongue-in-cheek brass plaque noting the piece is the work of Van Clomp (1455-1507).
There is a restaurant on the Great Square of Kortrijk in Belgium named "Bistro Allo Allo" after the series. The inside of the restaurant is decorated with black and white photos of a few of its most hilarious scenes.