Which reinforced maybe the most important of the four rules. Never point the muzzle at anything you do not intend to destroy. I had one once where I actually forgot to remove the mag on a 1911. Racked the slide, dumped the chambered round, pointed the gun at a concrete wall in my basement, and pulled the trigger. The only thing I can recall is, that mag was the only mag I owned without an extended floorplate. Poor excuse but that's all I got. I thank Hickock45 for his videos where he always clears his gun by pointing it downrange and pulling the trigger before returning it to the table.
Sometimes accidents make us smarter.