The List Universe has a neat countdown of the top 10 strange phenomena of the mind. For example:
3. Capgras Delusion
Capgras delusion is the phenomenon in which a person believes that a close friend or family member has been replaced by an identical looking impostor. This could be tied in to the old belief that babies were stolen and replaced by changelings in medieval folklore, as well as the modern idea of aliens taking over the bodies of people on earth to live amongst us for reasons unknown. This delusion is most common in people with schizophrenia but it can occur in other disorders.