Newer Super Mario Bros. Wii/Unused Level Features
Random leftover tiles outside of view will probably not be documented here unless they are particularly notable.
Inside the bonus area in Yoshi's House is some hidden pixel art and defunct Magic Platform sprites.
1-1 Palm Beach
Right under the first entrance in 1-1 - Palm Beach is a muncher, which don't appear in this level at all. There is also a Message Box and it's Manager.
This is Yoshi's Island.
B-2 Troopashell Track
Under each of the two Stalking Piranha Plants to the right of the second Star Coin is a platform that is set to be destroyed when the plant ontop of it is defeated, probably to prevent the player standing on it, and being saved from the pit and possibly getting trapped.
7-! The Blue Switch Palace
Under the room with the Hammer Mario blocks in 7-Switch Palace is some hidden pixel art of a hammers.