- Leonardo da Vinci once declared perpetual motion machines poppycock, on par with alchemy. But that was only after he tried to make one. His notebooks are littered with failed attempts.
- In 1989, chemists Martin Fleischmann and Stanley Pons claimed to have achieved cold fusion, a room-temperature fusion reaction, with cheap lab supplies. But no one has been able to recreate it.
- Medical doctor Randell Mills alleged in 2005 that he had created hydrinos‑‑hydrogen fuel with 1,000 times the energy of coal. But their very existence, physicists argue, would violate the laws of quantum mechanics.