the fork bomb, do it ;)

$ :(){ :|: & };:
\\_/| |||| ||\\- ... the function ':', initiating a chain-reaction: each ':' will start two more.
 | | |||| |\\- Definition ends now, to be able to run ...
 | | |||| \\- End of function-block
 | | |||\\- disown the functions (make them a background process), so that the children of a parent
 | | |||   will not be killed when the parent gets auto-killed
 | | ||\\- ... another copy of the ':'-function, which has to be loaded into memory.
 | | ||   So, ':|:' simply loads two copies of the function, whenever ':' is called
 | | |\\- ... and pipe its output to ...
 | | \\- Load a copy of the function ':' into memory ...
 | \\- Begin of function-definition
 \\- Define the function ':' without any parameters '()' as follows:

Prevention

Add a limitation in the limits /etc/security/limits.conf