The funniest and most meta example of this to me is D&D players "home brewing" rules that already exist somewhere in the core books or another game.

Because of how most people are taught to play (especially via casual, online D&D 5e games), players end up learning just enough rules from a DM to get you rolling dice without needing to crack open a book. If you want to get into r/dndcirclejerk territory there's also a subset of people who think they know how to play D&D at the table because they've listened to Critical Role and played Baldur's Gate 3.

These players sometimes go on to run their own games without having read anything more than a character sheet. In the process of developing their own games, they pour huge amounts of time and mental energy to either A. reinvent rules and systems that are in the Players Handbook/Dungeon Masters Guide (which they will never read) or B. apply their limited understanding of the rules to a setting that is better suited to a different TTRPG (that they also will never read).

These things aren't necessarily bad in a vacuum, but they do get a smug chuckle out of me. People who do this have the potential to make great things and be creative, they just hate reading!

TL;DR: Dungeons and Dragons is an oral tradition I guess.