Logged into a Unix machine? Does “ulimit -a” say that the max number of user processes is unlimited? Then type the following to crash the machine (provided it uses bash, or another shell with compatible syntax): 

:(){ :|:& };:

What does it do? It creates a function named “:”, which calles itself, and sends the output to itself.

You’d better find out how to protect your machine if it also states that the max number of user processes is unlimited. Even a hight limit can be dangerous, such as 15000.