We've all heard the old saying "what goes up, must come down." If you've never thought about this in relation to firearms, the same is true for bullets. If you shoot a bullet up in the air, it will come down somewhere and often with enough force to kill.
On New Years Eve 2016, a Texas legislator was hit in the head by a "stray" bullet. Thankfully he wasn't killed. In this instance, it is likely the gun owner was shooting his rifle into the air to celebrate the New Year. This happens far too often. Many Richmonders will remember 2013 when a seven year old boy was killed with a stray bullet while walking with his father on the 4th of July.
Neither of these incidents should have ever happened. The first rule of safe gun handling is to ALWAYS keep your gun pointed in a safe direction. "Up" is never a safe direction because you cannot guarantee where your bullet will land.