This seems obvious and if we sit down and talk about it, everyone will agree. However, this is the rule most often broken. Practitioners are not meaningfully engaged with what they are doing, and they move their body without realizing it changes the direction the gun is pointed, crossing the muzzle through things they don’t intend to shoot.
This is most common for new shooters who have yet to internalize a system of gunhandling. However, I also see very experienced practitioners do similarly foolish things, like an LE firearms instructor who center punched himself in the hand with a 1911. He obviously didn’t believe all guns are always loaded, and when he handled “unloaded” or “safe” guns he felt it was acceptable to point the gun at himself and pull the trigger. This sounds foolish, but don’t get smug and think you will never do it.
If you believe you will never do it because you are “smart,” then you are probably arrogant enough that you will eventually perform some negligent act with a gun. If you believe that you will never do it because you have embraced and submitted yourself to a systematic way of handling guns that keeps you from doing foolish things, then you are probably right