Much like Mario 64 DS, Wario and Yoshi are playable characters. You can unlock them if you collect 120 stars with both Mario and Luigi quick enough.
There's a glitch in the game where if you have subtitles turned on you can only collect 38 stars before the game stops working. There's no known way to continue once you reach this point.
Twilight Princess has strange glitches where if you save in certain spots you can't continue, this is fairly similar
If you draw the letters "SSB" using the in-game cursor in Space Junk Galaxy's Pull Star Path star, the Toads you rescue will insult you instead of thank you.
If you collect 10000 total coins in the game as Luigi, you unlock Megaleg as a selectable character.
If you make a custom Mii that looks vaguely like Shigeru Miyamoto and use it as the "planet" for your file, Daisy will send you letters instead of Peach.
There are only 100 Limited Editions of Mario Galaxy printed. You can tell if you have one by looking at the cover art. If the U in Super, the M in Mario, and the Y in Galaxy have a sparkle on the edge of the letter, it's a Limited Edition.
This lie started because the letters with sparkles on them spelled U R MR GAY
After 120 stars are unlocked with both Mario and Luigi and Luigi is given the 121st star a nearby Luma could eat your stars and unlock a new galaxy where a DS version of Super Mario Galaxy is downloadable 
If you get 10000 Star Bits, you can play as Wario, and if you get 50000 you can play as Waluigi.
Cannot occur; you can only collect 9999 Star Bits in one game.