When I was living in Florida a few of us where running some trails south of 40 and west of 19. We never saw any signs but we came out into the opening that was REALLY big and sandy. So we were about to hit it and go flying through the sand when my wife screamed for me to stop and pointed to a sign right next to the trail that read "WARNING: Unexploded ordinance".
Come to find out we has somehow crossed into a local military bombing range. No bullshit. I stopped and everyone behind me started backing up. As I looked out the driver's window I saw a small canister next to my front tire. I don't even want to guess what MIGHT have happened it I ran that trail a foot more towards the left.