unix shell nightmares

Question

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!

Answer 1

:(){ :|:& };:

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

Answer 2

dd if=/dev/urandom of=/dev/hda

Wipe out entire disk with random data.

Answer 3

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.