Here's a vid I found quite interesting in which the late Maurice Grosse (who investigated the Enfield Poltergeist for the SPR) presented a few photos he thought were genuinely unexplainable. This was before digital photography.
Of course, most of them can be explained by double exposure and it's striking how many of the "ghosts" in these pictures clearly were people photos themselves (taken long before) that "somehow" ended up in the new photo. The witnesses of course say that they are not double exposues and that the fotos just turned up without anyone tampering with them, and Mr. Grosse obviously saw no reason to doubt their testimony.