I believe either my brother or someone he knew once wrote a program that spawned lots of child processes. He did that to test a scheduler or something like that, but it got a bit out of hand and swamped a major server in endless processes. Admins weren't pleased. But also not too upset, because they approved of students experimenting. We had pretty cool admins.
A function called ':' is defined. In its body, it calls itself twice at the same time (':|:') (piping the output of the first call into the second, which doesn't do anything useful) and sends these calls to the background ('&'). After function ':' is finished being defined, it is called.
The first call spawns two clones. Each of those spawn two more. Etc.
:() defines a new function called :
{ :|:& } is the body of the function, where we call the : function recursively, piping (|) its output to another call to :, then backgrounding the whole thing (&)
; indicate the end of a statement and the start of a new one
: and finally the last : calls the function we defined to start the chain
Essentially each time the function is ran, it creates 2 new copies of itself, which each create 2 copies of themselves, etc. until your OS stops responding and crashes.
Nowadays many shells recognize this particular fork bomb and refuse to execute it
