Emacs just closed with 4 running shell buffers

["Chris Moore" <christopher.ian.moore@gmail.com>]

I don't know what I just typed, but somehow I managed to lose 4 large
shell buffers.

Usually when I have inferior shell processes running and hit C-x C-c,
I am prompted "Active processes exist; kill them and exit anyway?"

A few minutes ago I had 4 running shell processes, one of them was
part-way through a build, I typed a few characters by mistake and
Emacs closed down completely.  I'm not sure what I typed, but I
certainly didn't type "yes RET", which I thought was the only way to
lose my shell buffers.  I didn't kill all 4 processes, either.  I was
holding the control key at the time, and could well have typed
something involving C-x C-c.

I see that C-u C-x C-c will silently save and quit, but even that
prompts about running processes.  So what happened?  And can I stop it
happening again?