A many years ago, when I was young and foolish, I was exploring unmarked dirt roads on Cape Cod (Massachusetts), when I came upon a sign facing the other way. I stopped the car before I got to the sign and got out to see what it said.
It said: "Caution! Do not go past this sign! Active artillery range!"
The scary thing is that the sign was clearly referring to the parts I had just driven through, even thought there had been no similar sign anywhere earlier.
I turned around and went back the way I came, and didn't encounter any unexploded ordnance on the way out.
I have a snapshot somewhere of the sign with my car behind it. If I can find it, I'll post it.