In The original Paper Mario (N64), during the cutscene where Peach's Castle gets pulled from the ground and up to the heavens, you can actually barely see Goombario running away from the castle.
Here's a picture btw:
Mini Bowser, Mini Bowser Jr., and some giant Dry Bones heads in the background of Dashing Desert are the only Koopa-related things to appear throughout the Mario vs. Donkey Kong series. And Goombas don't appear in any capacity!
The term "Megavitamin" is hardly used in the Dr. Mario games. In every manual or tutorial for every Dr. Mario game that I can find, it's either "vitamins", "capsules", or "vitamin capsules". The modern games even used "capsule". The only time Megavitamin is ever used in the series is in Dr. Mario 64, which only appears in Story Mode. We likely took this for granted, because (1) Super Smash Bros. series widely used the term to refer to the capsules, and (2) the Mario Wiki used this term to refer to the capsules at the official page. Super Smash Bros. is even the more popular series, so it's probably why people assumed they are called Megavitamins.
As far as Mario games go, Dr. Luigi is the only game that celebrates Year of Luigi, and somebody on Discord joked that a 100 years later, historians would only know it existed from this game. I believe that this is universal across all localisations. Here are a few segments where it is acknowledged:
(The first image is one of the splash screens, after the game loaded)
Also, in Operation L (the mode with L-capsules), clearing a line will cause Luigi's cap (as depicted in Year of Luigi) to pop up. The higher the chain, the larger the hats are:
The music of the Cups in Mario Kart Arcade GP 2 that also appeared in the first game actually differ in both games. As an example, the beat in Mario Cup changes from a kick to a faster more breakbeat-like sound, and a section is added at 1:26 before looping.
Koopa The Quick might be based on either we wouldn't expect a turtle to be "quick" or the classic The Tortoise and The Hare story, especially considering they were considering his role to be taken by a rabbit in development
I like how in the Mario Tennis Bloopers, characters sound different in the retakes. I particularly like how Waluigi's "Wait for me!", normally emoted with urgency in the opening, is a lot more hasty and careless in the blooper version. He even goes like "nenenene, ah wait for me" like he's so sick of these retakes.
Mario Party 10 E3 2014 footage. At the 3:35 mark, they play the minigame Cheep Cheep Leap, and you'll notice that the music for this minigame is different. Originally, this minigame was going to have the music Gentle Sea Breeze play. In the final product, the Starman theme plays instead.
And you know what? I would've preferred that they didn't change the music. Gentle Sea Breeze is one of my favorite minigame themes from Mario Party 10 while the Starman theme is super repetitive, and to have to listen to that for 30 seconds is just ugh. It's one of those themes that you've already listened to a thousand times and you just get tired of it (I do, at least).
The music from Mario Party 10 that plays when you finish a round of Bowser Party is the Music Box track from Super Mario Bros. 3! I'm a big Mario Party 10 fan and have played Bowser Party plenty of times before, how have I never noticed this before? Probably because I don't pay much attention to SMB3 or its soundtrack and forgot that track even exists.
Wow, look at Mario Party 10 referencing the old classics!