Is it possible to make an homebrew application that silently change your mii name ? That would be funny, you open the homebrew, and boom, your mii name is "anus", a lot of fun
If it's not possible with miis, you could do it to pokemon save games, like a homebrew save editor with fake options such as "Fill box 1 with Hoopa", choosing any option would rename the player name to "idiot" or "trolled" for example, and when the user would try to restore his backup through the homebrew, a trollface appears, and finally after multiple tries it would finally restore the backup file...
Or a homebrew that show a fake brick message like "Nintendo have blocked your console due to an illegal application. Contact the support at [support url] for more information.", stuff like that
If it's not possible with miis, you could do it to pokemon save games, like a homebrew save editor with fake options such as "Fill box 1 with Hoopa", choosing any option would rename the player name to "idiot" or "trolled" for example, and when the user would try to restore his backup through the homebrew, a trollface appears, and finally after multiple tries it would finally restore the backup file...
Or a homebrew that show a fake brick message like "Nintendo have blocked your console due to an illegal application. Contact the support at [support url] for more information.", stuff like that