Mythbusters did a bunch of tests on falling bullets, and a bullet in free fall doesn't have enough power to kill a person. The problem is when people don't shoot straight up and instead fire at an angle, which causes the round to take on a ballistic tajectory where most of the momentum is still going forward and the bullet is going at a high speed.
A bit old (back in page 23) but here I go...
My school is located on top of a hill, and just below the hill is an army base. Coincidentally, the school field is placed almost perfectly in-line with the army firing range.
One of my good friends one day was playing in the field during lunch break when he felt something as if someone had punched him hard on his lower hip. He went up to the field supervisor who discovered that he was bloody all over.
Luckily no vital organs were hit, and there were practically no threats to future health, but he was taken to the hospital to get the bullet removed. He still has the bullet stored in a little case on his home desk, and it looks like it was fired from a rifle of some sort.
The school contacted the army right after this incident and it turns out the military was doing some drills on practicing firing on far targets, in this case, a red flag 300 feet short of the field.
Yes, both the school and the army have taken measures to prevent another incident like this - the school closed the field during lunch, and the military is (hopefully) no longer practicing on the red flag.
Just a story.