In Super Mario Galaxy, a planet can be seen in the distance in Good Egg Galaxy that should not exist. Attempting to fly to that planet makes it transform into a different one when Mario gets close. This is actually due to a completely wrong low-polygon model being assigned to it.
Comments
The Deep Dark low-poly model can be considered unused, since the player is never far enough to see it during regular gameplay.
It's an interesting look at how the game changed but some things got left behind