Nibbler's shadow

The shadow of Nibbler is a recurring theme on Futurama as well as important story arc. Originally it appeared in "Space Pilot 3000", but it should not be until "The Why of Fry" it was fully explained.

Development
For the production of "Space Pilot 3000", the writers made a point of adding secrets they would discuss later on, the most important - if not the only one - was the shadow of Nibbler. Nibbler was not introduced until "Love's Labours Lost in Space", and the shadow did not make a "reappearance" until "The Anthology of Interest I", though technically, in the what-if scenario, it wasn't actually there, explaining why Philip J. Fry did not fall in the tube.

Nibbler's truth was first revealed in "The Day the Earth Stood Stupid", but it was first in "The Why of Fry", Fry finds out the truth. Where in a flashback on display in the Infosphere, he sees that Nibbler there pushing him in the tube. However, he returns in time to intercept Nibbler, but decides himself to push himself in, so this additional shadow of Fry appears in a flashback in "Jurassic Bark", which aired prior to "The Why of Fry" giving new confusion to the shadow.

After the end of the original run, not much speculation on the shadow was left, however when the show came back as films, the shadow made a return by the very end of Into the Wild Green Yonder on the ship, before they fly into the black hole.

New speculation
With the new shadow, fans are once again getting on speculation, especially with the confirmation of the return of Futurama with season 6, where they expect the shadow to be once again addressed.