Ne prends pas la mouche.*

Graham Greene once said that France is the only country in the world where waiters and prostitutes don't hesitate to correct your grammar.

I have just finished reading John Baxter's "A Pound of Paper".  He says, "One of the first things I'd learned about the French is that they correct your grammar.  It's a national tic, like the way the Japanese bow.  They even joke about it themselves.  In Jean Renoir's movie, The Vanishing Corporal, Jean-Pierre Cassel is being helped to get out of a German prison camp by the local dentist's daughter.  Even as she outlines his escape route, he corrects her French."

In a footnote he adds:
"There is also the joke about the English tourist in the French café.

Tourist: Waiter, there's a fly in my soup."
Waiter: M'sieu?
Tourist: In my soup.  A fly.  Un mouche.
Waiter: Une mouche, m'sieu. It's feminine.
Tourist (peering) : Blimey, you've got good eyesight.

*Ne prends pas la mouche = Literally, don't take the fly.  It's an idiom which means "Don't get upset, don't be offended."