Piranha Plant save file corruption glitch
The Piranha Plant save file corruption glitch in Super Smash Bros. Ultimate, as its name implies, will corrupt the save file if performed. In order to execute this glitch, one must enter All Star Smash as Piranha Plant. Once this is done, there is a chance of the save file corrupting.
This glitch was introduced in 2.0.0 with the introduction of Piranha Plant. After being made aware of the existence of the glitch, Nintendo made the statement that they were unable to replicate the glitch in their testing. It was soon realized that many victims of the glitch used similar micro SD cards, which led to the theory that the game reacting weirdly to the micro SD cards under specific circumstances is the cause of the crashes.
A similar glitch was also discovered with Duck Hunt and Mii Swordfighter where using Wild Gunman against Gale Stab will corrupt the players save files, meaning that these two glitches may have something in common.