Over the years, a lot of people have claimed to have invented a perpetual motion machine. John Keely was one of them. Obviously, he hadn't invented any such thing, but here's how he managed to string people along for over 25 years.
A little perpetual motion device called the Feynman-Smoluchowski Rachet, also known as the Brownian Ratchet, was an early attempt at defying the laws of thermodynamics. It failed, but it was an entertaining failure.
Werner Heisenberg's uncertainty principle tells us it's impossible to know simultaneously the location and the velocity of a subatomic particle. It's one of the main ideas that famously prompted Albert Einstein to declare God doesn't play dice with the universe.
Physics is defined by its symmetries, from thermodynamics laws like the conservation of mass and energy, to the principle that the universe is basically the same all over. Symmetry can also suggest some truly bizarre ideas. One of those ideas is time crystals.