Super Mario 3D Land
Super Mario 3D Land
Mario Part 5 - Did You Know Gaming? Feat. MatPat from Game Theory
According to Kenta Motokura, the Tanooki-tailed Bowser is actually a Goomba using a Super Leaf. This was done as an homage to the original Super Mario Bros. game.
Contributed by GamerBen144
According to Koichi Hayashida, Cape Mario was originally considered as a power-up for the game before Tanooki Mario was ultimately chosen.
After the release of the game, the animal rights group PETA created a website and Flash game called "Mario Kills Tanooki." The game involves a skinless raccoon dog chasing Mario, who is wearing the suit. According to Eurogamer, Nintendo later made a statement saying:

"Mario often takes the appearance of certain animals and objects in his games. These have included a frog, a penguin, a balloon, and even a metallic version of himself. These lighthearted and whimsical transformations give Mario different abilities and make his games fun to play. The different forms that Mario takes make no statement beyond the games themselves."

PETA later stated that the game was "tongue-in-cheek", and intended to draw attention to real issues.
Contributed by GamerBen144
Unused audio of a Magikoopa Laughing can be found within the game's data.
Contributed by CuriousUserX90
Two alternate HOME screen jingles can be found in the game's files. The first is a shortened version of the one used in the final game, and the second is the first 6 notes of the Super Mario Bros. theme. The latter was reused in Animal Crossing: New Leaf for when the player puts on Mario-related clothing.
Contributed by Boyobmas
You can make any dandelions blow away and collect any coins that were hiding by blowing into the 3DS microphone. You can also blow away any ink on the screen left by the Inky Piranha Plants.
Contributed by KidDivinegon
It's not possible to fly in Tanooki form, but during the credits, Mario and the Toads appear to be flying while wearing the Tanooki Suit.
Contributed by Tuli0hWut
While the ending of the Bowser's Message fanfare from Super Mario 64 is used when Mario is shocked by what one of the flying letters has, there's also the first half of the fanfare left unused in the game.
Contributed by Boyobmas
Despite not appearing in-game, a sound effect for Yoshi from Super Mario World can be found in the game's files. In the pre-release trailers, the sound can be heard when Mario jumps on a note block.
Contributed by Boyobmas
Following the events of the massive earthquake that devastated Japan in March 2011, the development of Super Mario 3D Land shifted focus entirely. Nintendo's primary goal became making a game that could bring happiness to a tragedy-struck country.
Contributed by SideSmash
If enough time passes at the end of World 4-4 and at the beginning of World 8-4, a ghost appears.
Contributed by Funland47
World 5-2 is designed in a Legend of Zelda format, using a top-down view, lots of rooms, and no stompable enemies. Also, if you light all of the torches in a certain room using a Fire Flower, a secret door opens and a Zelda sound effect is heard. The level was designed in honor of LoZ's 25th anniversary, hinted at by the level number: "5-2" is "25" backwards.
Contributed by WiiWizard
In World 1-3, if you look up with the binoculars and wait, a UFO will fly by.
Contributed by DidYouKnowGaming