Here is a link to the background to the campaign of the Vercors, one of the most desperate battles fought by the resistance during the liberation of France.

Apparently, the order was given, "Bouclez le Vercors, le Jour-J" (raise, or call our, the Vercors on D-Day). Somewhere, this was taken to mean D-Day for Operation Overlord, the Normandy landings, rather than that for Operation Anvil, the later landings in the South of France. As a result, the liberated area on the plateau of the Vercors was overrun by German troops before help could reach it, and this was accompanied by sickening atrocities by SS troops.