Yume Nikki contains some unused spirites.
Mars
There is an unused door graphic in the tileset of Mars-san's room. It fits perfectly on one of the walls, but it just isn't there.
FC World C
A huge part of FC World, known as FC World C, is unreachable. There is also a smaller island with four statues that cannot be reached in any way.
Graphics for three different FC world trees exist, but only the one on the right is actually used.
There are six more unused tiles, and they appear to be drawn in a different art style.
Madotsuki's Room
There is a cracked window: 
And a round pillow: 
Puddle World
A Japanese Torii gate appears in the tileset.
Witch's Island
The pond on Witch's Island sometimes contains some sort of sea creature. If you noclip (preventing the player character from being obstructed by objects) in his pond and interact with him, he will make a strange noise. He can also be stabbed with the Knife effect by noclipping in the pond.
The Snow World
A pink gate. Note that the Pink Sea shares the same tileset as the Snow World. Perhaps this was meant to be the entrance to the Pink Sea, or it might has been scrapped because it didn't fit in the igloo. It appears in some other tilesets as well.
The Lost Party Member
Another Party member can be added to your party, but this never happens in the game.
It's nameless and there are no sprites associated with it. But having it in your party causes Several scripts in the game to change.
- All sane and insane Toriningen disappear. The Toriningen party and the nice lunatic in the Mall are not removed.
- The top-left cupboard in the Guillotine Room will disappear. If you stand on it's former tile and press the action button, then you will end up in front of one of the eight cupboards in the Eyeball World. If you enter the world again, then the cupboard will have returned to its rightful spot. This works in the bigger version of the Guillotine Room, too.
- If you wake up, you will be unable to move. You can still open your menu.
- The ending is not changed at all.
This party member requitement prevents events that should be "called" from being triggered directly.