The types of story you talk about in the OP appear from time to time, more so lately with the internet. I expect there would have been more Piltdown Men or Tasaday had the internet been ubiquitous as it is today. Twain said, "A lie can travel halfway around the world while the truth is putting on its shoes." I think he was right.

An example that grew legs in North America in 1995 was Neil Armstrong's remark while on the moon. The story can be read at Snopes. The thing is, it's a great story.

The grandmother tale has had variant forms and they serve a social function: good overcomes evil thus making people who feel disenfranchised or powerless the opportunity for the 'empire to strike back', with a vengeance. Of course, the further away the story takes place the better it is. Concomitantly, the facts are more difficult to check.

It's a ploy not lost on politicians (if the last American election is anything to go by). The old "if you can't beat 'em with brains, baffle them with BS."

Once again, my apologies for derailing the thread.