In the 1999 movie Fight Club, a character makes subtle appearances whenever "The Narrator" in the movie talks about his insomnia. During an office scene at roughly 04:07 as the narrator describes everything as being a "copy of a copy", one of the main characters in the movie, "Tyler", appears in a single frame for the first time.
To capture his appearance(s), you have to slow down your playback video speed as the narrator is talking then pause it at the right moment. In doing this, the voice audio of the narrator slows to an intoxicated, drugged, almost jaded state of being and as such, the narrator's subconscious in which Tyler lives. Not sure if this was the intention of the director, David Fincher. If it was then that's some insane detail, way ahead of his time.
"Tyler" is revealed by the narrator at roughly 04:07 of the film