I remember taking my young family to see grandma (DW's Mom) in 1980 and on the way back home to El Paso where I was stationed, we stopped at the south rim. We visited the rim by the visitor center (I think) and I started down a path that took me below the common area that most visitors were standing at. In front of me on this trail was beautiful views and about a 1000 foot drop straight down, so I was careful. I was standing there looking out and all of a sudden a fist sized rock flew by my head missing me by inches. If it had hit me I would have been knocked out and also knocked over the edge into that 1000 ft drop. I started yelling that there were people down here and to stop throwing rocks, assuming it was a youngster.
That was scary and comes back to haunt me periodically, like when I read GC reviews ;-)