There is a community that believes Eminem died in 2006-2007 and that his current manifestation is an imposter/doppelganger.
There are still people who believe that the Sun revolves around the Earth.
Aristotle believed the heart provided the ability to think, and the brain was just a heat sink for the heart.
Believing your memory is good will help you have good memory
There is a religion called Christian atheism. Practitioners believe in essentially the same things as traditional Christians except that the Bible is completely metaphorical and that God and/or Jesus is an allegory for human morality rather than a real being.
There is a subculture of people called Otherkin who genuinely believe they are not humans, either in a spiritual sense or a physical sense. These people may believe they are mythical creatures (elfs, vampires, etc,) animals trapped in human bodies, or a living incarnation of a fictional character, among other things.
In rural areas of India, a bizarre phenomenon has cropped up where those bitten by dogs believe they’ve been impregnated by them and puppies are growing in their bellies. Puppy pregnancy syndrome is a disturbing form of mass hysteria that is most prevalent in those with little education. Sufferers claim they can feel the puppies moving around inside them and often revert to bizarre behaviors like barking. Instead of seeking medical treatment, these people often seek out the help of a witch doctor—who gives oral treatments designed to dissolve the puppies, which are then “passed normally through the digestive tract.”
Some Matrix fans believe that Neo’s special powers worked in the “real world” because the real world was another simulation, suggesting an Inception-esque “Matrix within a Matrix” system.
There is a belief that a person only needs water, air, and light to live. It is called a breatharian.