There are a few unused sprites in the final game that can only be found by hacking.
Unused sprites in level
1A: Classic Piranha Plant - Works just like Piranha Plants from earlier Mario games. A bug causes the vine to use the wrong palette and tile.If you use it in a a hack, please use ExGFX to change the fish to a vine.
5E: Orange platform, goes on forever - If sprite buoyancy is disabled, moves forward without end. If sprite buoyancy is enabled, it acts like the regular orange platforms. This sprite can activate Fast BG Scroll. If you use this in a level, use sprite memory 01, others will make its GFX mess up a bit.
7E: Flying red coin - A red coin with wings that is worth five regular coins.
7F: Flying yellow 1-UP - A yellow 1-Up Mushroom with wings.
88: Cage - The cage is an unused sprite and an unused layer 3 image. The sprite was assumed to be garbage until it was discovered to turn into a solid cage for use with the layer 3 image. This cannot be implemented in a hack without messing up the castle's window GFX, due to the fact that it uses the layer3 tilemap. Also, It is basically useless, since it causes gameplay and graphical glitches.
96: Chargin' Chuck clone - A clone of Chargin' Chuck that appears to be unused. It acts exactly like a Chargin' Chuck.
E4: Swooper death bat ceiling - Creates a bunch of blue Swoopers that hangs from the ceiling, flies down and then back to the ceiling. These enemies can't be defeated. Because this sprite isn't fully coded, this sprite uses wrong graphics. To correct their graphics, change 0x17FBB to C0 by using a hex editor.
EC: Unused autoscroller - This is an autoscroller that starts with a horizontal layer 1 level, then switches to a vertical layer 2 section.
Floating skulls 2 - Creates six floating skulls. Stepping on one skull moves all skulls after (but not before) it to the right. (Used by giving a sprite the ASM pointer "87D9") Also note that these skulls will not keep moving if Mario gets off of them, and that the skull that is holding Mario will not move. Another thing to note is that if the skulls are vulnerable to fireballs and Mario shoots one at it, the skulls will move in a pattern exactly like sprite 61.(correct floating skulls)
(list is incomplete)
Unused sprites in overworld
Lakitu - A Lakitu that flies around and follows Mario on the overworld screen. This sprite cannot appear in submap.
Blue Bird - Acts much like the lakitu, but this sprite can appear in submap.
Piranha Plant - A small (classic) piranha plant.
Koopa Kid - Runs left, walks slowly back. If placed on a castle it will take Mario into the level when he touches the sprite. Technically it was on the map. However, it never appears without hacking.