The terminal velocity of a penny is about 60.3 miles per hour (I think it was). It does not have the force to break bone or usually even break the skin. On Mythbusters (on the Discovery channel) they rigged up a staple gun to shoot pennies at terminal velocity. They also shot eachother with it and only came out with a few bruises.
They also rigged a rifle or shotgun or some other form of firearm to shoot a penny at about the same speed as a bullet. When shot at the same speed as a bullet, it had the same effect as a bullet and shattered the dummy head they were using to prove that a penny *cannot* fall off the Empire State Building and kill people at the bottom or wedge itself in the concrete unless somehow someone shot from the top with a gun.
Ok, now that I\'m done being a geek, I think it would be cool to have random little cave-ins or such. After all, the game *does* take place in a cave, and anything with a roof could send debris down in some way.
As for what is falling, maybe some things of value such as gems and ores, but a greater number of just random bits of rock than stuff worth anything. Also, though, the falling rocks would probably be concentrated mainly in one place, which would be the weakest part of the roof/cieling/whatever, and so most of the people would probably just get hit on the head with chunks somewhere between the size of a baseball and random pebbles (hopefully your char would be on the side away from all the huge rocks, that is). Even then, stone forms in layers, so it\'d be likely that the parts falling would be long, flat, and rather skinny unless they were a stalagmite/stalagtite (I dunno which is which any more), making them probably just a few feet long and wide, and maybe up to one foot in height... Or maybe I\'m just overthinking things, so I\'m just gonna shut up now.