I seem to remember borrowing a PC (possibly a new-fangled 286!) from school one summer holiday when I was about 8 or 9, and doing some "tidying up"... which possibly involved the use of "format c:"
It didn't have any immediately deleterious effect until after I switched the machine off if memory serves, at which point it became completely apparent that I'd buggered it up big style - or that's how it looked to me at the time. I think the head teacher managed to reinstall everything from floppies and saved the day.
I remember that same gut wrenching moment of terror on only one other occasional: I was, for $reasons, merging two NDS trees within a school (one admin and one curriculum). I'd done the merge, and then started updating the ADMIN server from Netware 5 to 5.1 (again, $reasons - it was working fine, I should have left it be...) and somewhere through the install there was an option to either leave the disks as they were or format them. No prizes for guessing what I managed to do! I had backups, so nothing was lost except a bit of extended downtime, and I've learned to read things a bit more carefully since then, which is no bad thing.*
*Unless you ask me to check a document over, in which case I've a tendency to notice all of those little things you'd rather I didn't.