neat thing people might not be aware of:
If you load a DS game into a 3DS, and then hold start while starting the game, it will boot into the original DS resolution, for pixel-perfect unscaled accuracy.
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another nintendo easter egg: you can change the sound effects used in the gamecube logo screen. Holding Z on one to three controllers changes it to squeaks, and holding Z on all four controllers changes it to kids laughing.
Another another nintendo easter egg: when starting up a gameboy game in a gameboy color or gameboy advance, while still on the gameboy logo you can press a combination of A, B, and d-pad directions to select between different color palettes to play the game in.