My favourite is the story that there was mass panic over a radio broadcast of War of the Worlds where everyone thought a real alien invasion was happening. I heard this story as a kid and really thought this was a cruel prank played by the radio station.
In reality, they made it clear at the beginning of the broadcast, and twice during, that it was fictional. Not that many people were listening and most of the people who were, were aware it wasn’t real. A few idiots freaked out and it somehow turned into a story of mass panic. It was propaganda by newspapers to discredit radio.
I think that’s being unreasonably generous to the French. Their military was staffed with elderly officers, and they had very poor communications. They didn’t use radio very much and telephones were often cut, so they had to rely on motorcycle riders.
Also, for some reason they refuse to believe that the Germans would advance through the Ardennes, as they had the previous two times they invaded France.
Their army then essentially gave up, and they agreed to set up Vichy France, a collaborationist state that was very efficient at rounding up and deporting Jews to the gas chambers.
The French also refused to hand their Mediterranean fleet over to the British at Mers-el-Kébir, so the British sunk it rather than risk allowing the ships to be used by the Germans.