Drivers passing the camera in the town of Eaucourt-sur-Somme at over 50 km/h saw “F*ck you” flash up on the screen.
But the monitor was as courteous as it was rude – those going below the limit were greeted with “welcome to Moulin”.
The company that supplied the speed monitor, JCB, have apologised for the message, which could be seen for several days, claiming a “practical joker” was behind it.
JCB boss Jean Claude Bouton said a contractor had changed the text during testing to prove the text on the monitor was too small, but forgot to change it back after.
“I sincerely apologise, but that’s all I can do because the damage has been done,” said Mr Bouton.