In the Super Mario 3D All-Stars port of Super Mario Galaxy, the game's cutscenes were re-rendered into HD. However, an error slipped through where Princess Peach can now be seen T-posing during the game's opening sequence just after Bowser's airships drop their anchors into Peach's Castle.

The Toy Time Galaxy theme in Super Mario Galaxy and the Supermassive Galaxy theme in Super Mario Galaxy 2 may both be based on "Mario Syndrome", an alternative dance remix of the Super Mario Bros. overworld theme by Japanese act Bonus 21 that was released as a 12" and cassette single in 1986. Both themes feature similar instrumentation and compositional choices as "Mario Syndrome", though foregoes the 1986 song's dance elements.

There are unused animations of Mario performing punches and playing Tennis.

A large castle area called "Starmans Fort" is still present in the games files, but was never finished and went unused in the final game for unknown reasons.

The messages that appear when you lose a life in Super Mario Sunshine and Super Mario Galaxy differ between regional releases of the games. In the Japanese versions of both Sunshine and Galaxy, when you lose a life the text says "Miss!", but this was changed in the international versions of the games to say "Too bad!". The only difference is the Korean version of Galaxy, where the text was reverted back to "Miss!"

Due to an unusual mistake, people who preordered the game from GameStop received an automated phone call from them claiming it was a Mature rated game.

In the level Good Egg Galaxy, there's a sign post hidden behind the door that Mario can "Read", made to act as a message on the door. This was done by the developers as a shortcut so instead of having to program a readable door.

After collecting x9999 starbits, all of the coconuts turn into watermelons.

In the final chapter of Rosalina's storybook, in the French version on one of the pages, the line is different than compared to the version shown in the game in other regions. When translated, the line reads:

"I would like, once every hundred years, to return back to my blue planet, and doze on the knees of my father, stroking the mustache that was his pride."

The Dusty Dune Galaxy has a similar name to the Dusty Dunes Desert from EarthBound. However, this is most likely a coincidence.

The idea for Super Mario Galaxy simply came from the game's producer, Shigeru Miyamoto, saying to the game's director, Yoshiaki Koizumi "Hey, can you make something with spherical worlds?" This idea became defined over a long period of time and would eventually turn into Super Mario Galaxy.

Miyamoto was known to provide vague comments and feedback that not many workers at Nintendo could completely understand and that only Koizumi was able to figure out the context for.

Within the game's files is an unused land mine. Though it goes unused, by modifying the game to insert it back into a level, it appears to be fully functional.

A collection of realistic-looking bugs can be found in the game's files rigged and animated. It is unknown what these bugs would have been used for.

Rosalina was originally going to have a beehive hairstyle along with short straight-cut bangs, and long, curly locks in front of her ears. She also had more unique facial features, rather than looking similar to Peach. Her dress, however, was changed to look less like that of Peach or Daisy.

In the Rolling Gizmo Galaxy, there is a hidden cluster of Star Bits that resembles a Blue Rupee from the Legend of Zelda series.

An unused Luma called "Ticojii" is present in the game's data. Ticojii vaguely resembles Eldstar from Paper Mario, and the Millennium Star from Mario Party 3.

Koichi Hayashida, the Director of Super Mario Galaxy 2 and Level Design Director of Super Mario Galaxy, provided the voice for Major Burrows.

Yoshi was originally planned to appear in Super Mario Galaxy, and was featured in proposal documents for the game from 2005. However, he was removed due to the developers thinking that "it would've been too much all at once." They said that, "if you're going to put Yoshi in a game, he has to be a main element, but [Super Mario Galaxy] had spherical land forms and gravity shifts and lots of elements that were entirely new" and, "Even if we had used him, we might have only been able to use him on a single stage."

Despite this, it was decided early on in the development of Super Mario Galaxy 2 to include Yoshi in the game, because his control scheme could act as a new main element of a new game and a "multiplication" of the elements from the previous game. The Wiimote controls for Yoshi were also formed early in development, with Shigeru Miyamoto taking pride in the tongue pulling mechanic used for eating fruit, fighting enemies and flipping switches, saying it "isn’t like anything you've ever experienced before." The rest of Yoshi's controls were shaped based on internal feedback from "everybody's love" for Yoshi, listing the development team, Mario Club Inc., and Nintendo of America as primary influencers for what the image of Yoshi should be, but tried to avoid making him too powerful. The developers also had composer Kazumi Totaka record new voice lines for Yoshi for the first time in ten years, and was reported as feeling "a little uneasy" and worried that Yoshi would sound like he had aged, but these were not concerns to them and the recordings were used in the game.

Rosalina's Storybook was written by Yoshiaki Koizumi, who wrote it at night without anybody knowing what he was doing. The following day he presented the story to Shigeru Miyamoto, who was surprised when Yoshiaki told him that nobody else knew about it.

Strangely, when the player plays as Luigi in the mission "Luigi and the Haunted Mansion" the trapped Luigi will still call out for Mario. This could either be something the developers missed or the fact that he could have been expecting Mario to rescue him.