When I was at college, during the hot summers of the 1970s, a lot of the other lads on the course worked for the Coal Board. One of them mentioned that he was about to buy some new sunglasses. Another lad, who was very clever, but could be a bit naive, said "You won't need them down the pit!".
This started a chain of events over a few days, which ended up with him believing that coal is actually brilliant white when it's first dug out, only oxidising to the familiar black colour over a few hours - and the blinding glare in a coal mine was quite a safety hazard.
Every piece of evidence that he came up with for not believing this was shot down immediately. Miners are always pictured covered in black dust - this is because the dust has been on them for the whole shift, Bob. The reason they often have white rings round their eyes is because they've just taken their sunglasses off for the photo. Etc.
The clincher was when a photo was produced, showing a roadway underground. This had just been "stonedusted" - where white stone dust was sprayed over all the surfaces, intended to mitigate the effects of any explosions.
This sort of thing, but without the chap and the hose in the picture to give it away.
I know that he still believed in the 'white coal' two years later - maybe he still does?