Contributed by WhiteSamurott
In the Deep Dark Galaxy, on the shrinking planet , you can crawl into the blue house and see the inside of the of the planet. Also the walls are not solid on the inside, so you can walk out.
Contributed by Bean101
The Japanese Super Mario Galaxy announces "Star get!" when you get a Star, and "Grand Star get!" when you get a Grand Star. In the U.S. version, the grammar was cleaned up to read "You got a Star!" and "You got a Grand Star!" A similar change occurred in Super Mario Sunshine, where "Shine get!" was changed to "Shine!"
Contributed by game4brains
The battle against Megaleg is based on a scrapped concept of the final battle in The Legend of Zelda: Ocarina of Time, where Link would have to scale Ganon's (then-colossal) body in order to defeat him.
Contributed by MortalKombat2007
During the Spaceworld event of August 1, 2000, a tech demo called Super Mario 128 was shown featuring many clones of Mario. The tech demo also featured sphere walking which was later used prominently in Super Mario Galaxy.
Contributed by Charlie
There is a planetoid in the Space Junk Galaxy that looks a lot like the ship from Pikmin 2.