Never underestimate the lengths stupid people will go to
Mr. Holmes is basically correct, a proper nailgun has a catch on the front that has to be pressed against a solid surface before one can pull the trigger to fire a nail. However, in the two years or so when I built theater sets, I've seen and heard of many people endangering themselves and others through stupidity.
Some nailguns will let you pull the trigger back even when the gun is not pressed against a surface- and if the trigger is back the gun will fire whenever the tip presses against something. This is popular with some contractors because you just hold down the trigger and tap the front of the gun against whatever you need to nail, over and over again. Its fast, but dangerous- one stumble on the job, and the tip of the gun hits a human body instead of the intended target.
Its also not hard to imagine how a technically skilled (but otherwise stupid) person could jam or modify the catch on the front so the nailgun will fire any time the trigger is pulled.
Finally, I once saw a kid with a nailgun horsing around and pointing it at his co-worker like he was going to shoot him. It doesn't matter that its not connected to the compressor, it doesn't matter that it has a safety, it doesn't matter that there are no nails in it.
I don't think its a DIY problem, I think its a lack of sense problem. Read the user guide, know your equipment, and know your limitations, and no one gets hurt.