Today I was almost killed when couple of teenagers tossed grapefruit-sized rock off 30 foot overpass, barely missing me.
Heavily trafficked foot / bike trail that I ride all the time; there are no rocks, bricks, chunks of cement, cinderblock, etc. on the bridge so they had to have brought it with them; and they attempted to silently slink away when I hollered at them so it was absolutely intentional (not that 'trundling' is any better).
PSA: Keep an eye on overpasses ;[