In Paper Mario Sticker Star, there's this piece of paper with the code: "XD3R-B8HH-9ZR2-FL16". As far as I know, no one has deciphered what this means.
It's not a code to anything on the eshop, because those codes don't have Z in them, it's too long to be a friend code, and I don't think it's a Mario Maker 1 ID (Though if it was, we wouldn't be able to check. Also this game came out years before Mario Maker... so yeah).
Shesez (The boundary break guy) said "I've been told what it really is" on Twitter.
After a year of people begging him to say what it was, he responded with this:
Honestly this probably means he was lying, but even if he isn't we still have no idea what the code means.
Anyways, a Twitter user found that adding a ".com" to the code and searching it brings you to
a website a different code. However this website was created after Supper Mario Broth reported on the code, so whoever made it was probably just trying to make their own ARG or something. It's still pretty cool though.
Another Twitter user converted the code into binary, and it created an image (I recommend reading that whole thread):
And that's about all I could find about it unfortunately. For hated as this game is, I think it's actually really cool to have an unsolved mystery in a Mario game. One potential lead that I think is worth exploring is checking the Japanese version of the game and seeing if the code is there too. It might just be something added in the western localization that doesn't really mean anything. I'm not able to do this, but if anyone else can read Japanese and has a Japanese copy of Sticker Star, please check. It's in World 5-1.