This might apply if driving in Paris, Lyon and Grenoble this year, e.g. a trip to the alps.
“British drivers heading to the France capital are advised by the RAC that driving without a France ‘clean air’ sticker, called a Crit’Air vignette – which costs as little as £3.20 – makes them culpable for an on-the-spot fine of up to £117.”
http://www.rac.co.uk/drive/news/motoring-news/law-change-for-uk-drivers-in-french-cities/
It’s the wrong date for an April fool trick and the RAC probably wouldn’t do that sort of thing, I’d guess.