Perpetual Motion Machines.
What are some bad science hoax ideas?
The whole story of Piltdown Man is fascinating, and was only exposed as a hoax 40 years after "discovery".
All these new medicines with a fifty second disclaimer. My new favorite is one to cure laughing
Reactionless drives. The machine will go forward with no force going backward. There are many that have been patented, but none ever proven
Some years ago the Scientific American published an article under the heading "There is nothing new under the sun" in which they revealed that Alexander the Great had the first wrist watch in history.
It wasn't like a modern digital watch, or even one of those wind-up jobs with springs and winders and big and little hands sweeping around a numbered dial. It was a simple piece of cloth that his alchemists had soaked in photo-sensitive chemicals. He tied it around his wrist and its colour varied according to the amount of sunlight it received during the day, allowing him to know the approximate time.
It was called Alexander's rag time-band.
But the funny thing is that the French (who sometimes fail to understand American humour) published an expanded version of the story and quoted the original spoof in Scientific American as source material.
https://www.youtube.com/watch?v=OFrPlV1IbJI
ANY "miracle cure" ... They call 'em that, because there's no science behind it - which generally means it doesn't actually cure anything, either.