I'm Gonna Guess that they implement something in the 3ds firmware that when a flashcard is inserted in it, it overwrites the flash on the flashcard that damages the card where it can't be fixed. and can't be used anymore. it could also have a database stored for all ds titles and checks for original game data and header/signature and check if it's a genuine card by the game data on the card and the crc32 checksum of the card. and the database is updated through online system updates to play new games or when inserting new 3ds games and I already know they are made a Japanese game that damaged some flashcards when running as a rom on flashcards. I can't remember the game name just a theory tell me what you think?