Stride over that fence
This might be one of the nastiest device I've seen on the street (apart from barbed wires). I ran across this thing in Paris few weeks ago, not sure about what it's made for but the affordance for me is clear: don't put stuff in that street corner, don't stand there, and inevitably don't pee here. Why nasty? the pointy fences are definitely aggressive.
To me this is a nice metaphor for things such as DRM (digital right management) protections that are supposed to prevent you from doing something but inevitably it's cracked (or you can stride over that fence) and do what you wanted to.