What are the chances???

[Pa SxS]

I was at our local gun club early this weekend before actual shooting would start. We were talking in small groups in the parking lot when I heard a shot going off. I figured someone was testing his gun or shell. Turns out one guy dropped a shell on the gravel/stone parking lot and the shell went off. Luckily he wasn't hurt, just stung by the powder and shot. Nothing visible on his body. I've never heard of this happening before. The shell must have landed perfectly with the primer hitting the point of a stone. Look at the primer indentation.