unix shell nightmares

What are the worst/best unix shell nightmares? My favourites are:

• rm -rf / tmp/foo (deletes everything if you are root; typo :-( )
• rm -rf .* (deletes everything if you are root; not just all hidden files)
• Files in * that are used as parameters, e.g. after touch -- -rf, rm * eventually evaluates to rm -rf foo bar
• cat foo | sed 's/foo/bar/g' > foo (deletes foo)
• some_command > /dev/sda

WARNING: Do not try any of these or the following commands!

From stackoverflow

• :(){ :|:& };:
  

  Warning: DO NOT DO THIS

  This is a fork bomb. The equivalent of:

  while (1) {
      fork();
  }
  

  Only do it as a test on a machine you have physical access to. You need to be able to hard reset :D

  Johannes Weiß : I also thought about that one, but when do you type in that by accident ;-)?

  Sean Bright : You never mentioned "by accident" :)

  Scottie T : So for those of us who might just be curious enough to ssh into our boss' terminal and try that out, what does that do?

  Johannes Schaub - litb : Johannes, haha yeah i also thought whether i should answer with that :p though i would probably never write it by accident. but i think sean is right: you omitted to mention you meant "accident" stuff :)

  agnul : @Scottie: see http://en.wikipedia.org/wiki/Fork_bomb

  skinp : Classical Fork Bomb :D. Run "ulimit -u 30" if you want to try this one out... You'll limit the fork bomb to spawn only 30 process

  Sean Bright : I ran this while SSH'd into my home machine from work before I knew what it was. When I got home the fan in the machine was running so fast it sounded like a jet engine in my office.

  Johannes Schaub - litb : yeah haha i once run that code too, when i thought my machine won't crash and i could easily kill shell (oh no, i could not!) :p

  MatthieuP : I think this is lcamtuf signature, right ?

• dd if=/dev/urandom of=/dev/hda
  

  Wipe out entire disk with random data.

• cat /dev/random > /dev/hda 
  # oops, why don't i hear sound? oh noes, i meant /dev/dsp!
  

  The code above was given in a suse linux manual as an example why working as root is dangerous. I guess the user doing that mistake learned something for life.

  Joachim Sauer : that's why you should not work as root ;-)

  Paul Tomblin : saua, you wrote that while I was writing the same thing in the question.