Fregoli delusion, which is named after the Italian actor Leopoldo Fregoli, is a condition that leads one to believe a specific person is using disguises to impersonate others you cross paths with. The first recorded case involved a female theatre-goer who believed two of the actors were secretly pursuing her by disguising themselves as other people she knew or met.
Modern technology and communications have only added to the repertoire of strange delusions. Read about 15 of them in the linked post. Link -via the Presurfer
(Image credit: Flickr user Johnson Cameraface)