Not long ago, there were several discussions on UbuntuForums regarding a malicious command which was deliberately posted in the forums in order to make users damage their system if they typed it in a terminal using root privileges. The command is 'sudo rm -rf /' and means 'use root privileges to recursively remove, without asking for permission, the root directory / and its entire contents. I never tried it, but I DON'T recommend you to do so. It basically wipes out your Ubuntu (or Debian) installation. I think a workaround will soon be found for this, if it hasn't been yet.