Ghost Photos.

From the Top Ten Best Ghost Photo Ever Taken at The Knight Shift:

Freddy Jackson was a mechanic in the Royal Air Force in World War I. Freddy Jackson's squadron served onboard the H.M.S. Daedalus. Freddy Jackson was killed in 1919 when an airplane propeller hit him. Two days later when the squadron assembled for a group photo, Freddy Jackson faithfully showed up, grinning behind the ear of a fellow comrade. Guess nobody bothered to tell Freddy Jackson that he was dead. His face was widely recognized in this photo by members of the squadron.

See nine more eerie ghost photos: Link - via Seven Deadly Sinners