In the virtual machine you normally ran CMS but you could run anything. Some machines ran MVS.
To direct a command to the virtual machine itself you would prefix the command with a special character which by default was # but any chosen character could be the magic prefix. So #cp ... would be a command to the virtual machine.
Bored one day I wondered if a virtual machine could run VM itself, on the "second level". I booted it up, changed the prefix character to ! (so, !cp). I could create new virtual machines inside this new VM.
So, could a second level virtual machine run VM? I booted it up on the "third level", changed the prefix to @ (so, @cp)...
I got 8 levels deep. So, yes, VM could run VM, could run VM, could run VM... etc.
Game over. Time to start shutting down these embedded levels.
Out of habit I typed "#cp shutdown" ... and it did. The REAL VM on the REAL machine shut down. Panic run to the machine room to push the start button on the console.
Of course the system keeps a log ... and the other systems programmer showed up at my door ... and said "don't do that again".
Fun times.