Mario Paint
Mario Paint
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Updated versions of the game were distributed online via the Satellaview service in Japan; these versions support standard controllers as well as the Super NES Mouse. The "Yuushou Naizou Ban" version of the game also adds the picture "Dreaming of the Moon" by Mizota Hiroko of Tokyo, the winner of a contest associated with "Mario Paint BS-ban".
Contributed by MehDeletingLater
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If your regular and animation canvases are sufficiently large and detailed that the game cannot compress them into the cartridge's 32 kilobytes of SRAM, the Save Robot will display a rarely seen "DATA OVER FLOW" error, complete with a unique animation of smoke puffing out of its "ears" with a looping explosion sound effect.
Contributed by MehDeletingLater
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There is a fifth, unused icon for the Coloring Book that resembles a small toy bear. It is unknown what this icon would have done, but as the game's graphics are uncompressed and thus take up a significant portion of the ROM, the developers likely ran out of memory and were forced to scrap it.
Contributed by MehDeletingLater
By plugging a controller into the second port on a SNES, two extra features can be accessed:

•Pressing A/B/Start at the same time will start the game without having to click on Mario.
•Pressing A/B/Select at the same time will (after a short delay) load and display the currently saved drawing, animation, and music. If nothing has been saved, "NO SAVE DATA" will be displayed instead, and you must reset the console to return to the title screen.
Contributed by MehDeletingLater
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The mini-game Gnat Attack actually has a series of trophies that are awarded each time the game is beat. While not "unused", it is unlikely most players bothered to (or were able to) play through the game 15 times to collect them all.
Contributed by G-Haven
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There's a set of rather useful features that are, for some reason, disabled by default. However, they can be unlocked via a simple "code" on the title screen by clicking on the "N" in "MARIO PAINT" to start the staff roll and, when "PROGRAMMER NORIAKI TERAMOTO" appears, holding the right button and clicking the "N" again. By doing so, the player will unlock a series of available commands, most of which are utilized through the right button.

The list of (currently known) hidden features are as follows:
• Right-clicking anywhere on the screen will activate Undodog (with a few exceptions).
• Right-clicking on the canvas in the Stamp Editor will sample the pixel color underneath the cursor.
• Four new rotation options are available in addition to the old horizontal/vertical flip options (see screenshot).
• Pressing L + R + Start + Select on Controller 2 will reset the game (code will need to be entered again).
Contributed by game4brains
If the player clicks the R in "MARIO PAINT" on the title screen, a sound reminiscent of a baby laughing will play. If the sound is reversed and slowed down by 50%, it will reveal itself as a backmasked voice saying "Nintendo".
Contributed by game4brains
On the title screen, pressing the O will turn it into a bomb, which then explodes. After this, a short 19-note melody will play. This is Totaka's song, a song hidden by Nintendo composer Kazumi Totaka in almost all of the games he has worked on.
Contributed by DeerBoarDude