After purchasing a used copy of Paper Mario with some particularly odd save files, self-described Paper Mario-destroyer Stryder7x began to wonder: Exactly how messed up could he leave a copy of Paper Mario for someone else to find? Short answer: Very.
The long answer is far more complicated, and involves some specific terminology. Essentially, Stryder7x goes about leaving the four saves available in the game in unique "softlock" states. "Softlock" here meaning that he's triggered a certain series of events that'll cause the game to break indefinitely. It's commonly used in the speedrunning community to refer to an unintended error that effectively destroys a run.
Why does this happen? Speedrunners commonly use glitches inherent to the way games are coded to access places they shouldn't. In this specific case, Stryder7x is causing softlocks intentionally, and with utter precision. For example, he uses a series of glitches in Mt. Lavalava to cause the game to save right before it tries to play a Peach event that doesn't actually exist—leaving the game in limbo. Loading the save file will only bring it back to that point, meaning that save slot is forever broken.
In another example, Stryder7x returns to the biggest room in Mt. Lavalava and glitches out a save block so that it doesn't force the player to remain stationary while saving, which is typically the case. Through some careful manipulation, he sets it so that he saves immediately prior to causing a game over by jumping on an enemy. Reloading this save file simply repeats the action with no time to change it, and thus repeats the game over screen too.
Stryder7x suggests using this as an admittedly complicated way to leave an interesting set of save files on a game you sell or loan to others. Someone booting it up for the first time that's curious about the saves titled "DONT", "CHOOSE", "THESE", and "FILES" will be met with several peculiar scenarios that can never progress.
Game over, man. Game over.