What's the biggest mistake you've made on a server?
I think mine would have to be what happened to me today. I accidentally deleted /etc and didn't have a recent backup of it. Fortunately, I did have a backup of my nginx config and a very old backup from a previous install.
Thankfully I've never done rm -rf / except intentionally on a machine I was just messing around with. Mistakes happen to the best of us, so what's the biggest mistake YOU've made?