The problem is that there are no signs on the fence explaining why it is there, and the people who raised it are nowhere to be found, and the most likely reason for its existence is unreasonable paranoia.

> the most likely reason for its existence is unreasonable paranoia

Spoken like someone who's never walked across a minefield ;)

And farmers still plough up mines in the Somme, because they still have to plant next years crops.

On that subject, unexploded ordnance (UXO) is a real danger for the Crossrail tunnels deep under London:

http://www.londonreconnections.com/2012/a-legacy-of-iron-cro...

I can't link it (YouTube is blocked at work), but there was a BBC program from the 80's called "Danger: UXB" which is quite excellent, and nearly every episode is online.

It's mostly artillery, which is less likely to kill on a touch. Plus I'm sure one could spin an argument about this being an example of knowing how to break rules relatively safely, too.