One from a fellow student, who didn't know about fork bombs and put one in his .bashrc, following "advice" from a friend, he never figured out how to fix it and just reinstalled

On my part, it was a server install of YunoHost that I broke by trying to setup an app to use the LDAP provider. Since I needed the YunoHost LDAP password, I messed with some files, broke the LDAP config, but it turns out everything in YunoHost uses LDAP. Including your own user and its associated privileges. So the server was entirely broken, and it was impossible to restore backups because the YunoHost restore tool was also botched by the config errors