This image was published in 1975 by Sir Victor Goddard. It was originally taken in 1919 and shows Goddard's squadron. When developed, it revealed what appeared to be a ghostly face behind the head of one of the airmen standing in the back row, fourth from the left.
Upon seeing the image, members of the squandron claimed to recognise the face to be that of air mechanic Freddy Jackson, who had been accidentally killed by an airplane propeller two days before the picture was taken.
Jackson's funeral was held the very same day that the group shot was snapped.
Double exposure, maybe?