Updated: May 30, 2008
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.
The community's response to this was fast and efficient: in only several days, a huge number of users changed or added to their signature a warning saying to never use this command or any similar ones. A sticky thread explaining the dangers of using such a command was also available.
What amazed me was the solidarity and good intention of community members, which quickly mobilised and practically spread the word on not to use the above or any similar commands. I was really impressed.
The problem which arrives is that new Linux users are tempted to take such commands or scripts from all over the Internet and paste it brainlessly into the terminal, so they quickly complete whatever task they initially wanted to do. This I guess, should be treated carefully and probably such warnings on malicious commands should be available and easy to jump into, in order to reduce the risks at minimum.