ｗｈｏ ａｒｅ ｙｏｕ ｒｕｎｎｉｎｇ ｆｒｏｍ？
If you groundpound 13 times (a reference to Game Boy Camera, which is 13 letters) on the first level, you will go through the ground to fall in a dark room that has this error screen from Game Boy Camera set as a texture for all of the room's walls. The only way to exit this room is by restarting the level.
Super Mario 64 Galaxy
There are at least two easter eggs related to the number 8.
The second one is that, during the cutscene when you're floating in space at the beginning of the game, you can see our solar system (the real life one) in the distance. There are eight planets in our solar system, and you can see all of them.