The following is a very stupid bash command to run.
This is a fork bomb, so don't run it without having set process limits.
I was trying to understand how this worked, and I found the following blog that set me straight. What was tricking me was the colon... it didn't look like it should be valid but it is. Who knew? Well, certainly not me.
The site I note above helpfully suggests changing the colon to "bomb", which gives us:
bomb | bomb