Here's a really interesting bit of trivia about Super Mario Galaxy that you might not know. There's a single mushroom in the game that you can't access through normal means, and if that mushroom is removed from the game's code, the entire game crashes. Super Mario Galaxy, as well as its follow-up, need that mushroom in order to run properly.
It appears Nintendo has gone in and fixed the load-bearing mushroom for the Super Mario 3D All-Stars version of Super Mario Galaxy. While the mushroom still exists in the game's files as model data, it can be yanked from the game's code without causing an issue.
Why did this mushroom need to be present to begin with? There are some rumblings that an early version of Super Mario Galaxy had some sort of playable mushroom component. When Nintendo scrapped that and moved on, part of the code tied to the game remained tied to that mushroom, and removing it borked the entire game. Instead of trying to figure out the issue, Nintendo just hid the mushroom outside the normal methods of play, and called it a day!