I know of that happening, too. A few years ago, a guy near here was killed by a bullet on New Years Eve. After an investigation, they determined that it came from the sky, from some moron firing his gun on New Years Eve.

Mythbusters did a piece on the lethality of shooting guns into the air. If you shoot it straight up, and it falls straight down, air friction will reduce it's terminal velocity, make it tumble, and while it would hurt you, it probably wouldn't kill you.

The problem is that it is nearly impossible to fire it straight up. It is almost always going to be in some sort of arc, so it is never really in a position for terminal velocity, friction, and tumbling to mitigate its lethality. So it just travels normally, and eventually hits someone.