No, for the first part not proper “etiquette” but habit. And for the second part, just human stupidity or a mistake. If you remove the clip from the gun for the purpose of demonstrating that it is unloaded (such as, when you are handing it to someone for their inspection), when the gun is returned you typically put the clip back in. This is because 1) you’ve got it in your hand, 2) it belongs in the gun if you want to store it. However, the slide should be worked shut before inserting the clip, if you want to have a clip loaded but not the chamber loaded. If you were on the range shooting, or were a law enforcement officer who typically carried the weapon with a live round in the chamber, you would insert the clip, then work the slide, which results in a gun that is cocked and ready to fire, with a live round in the chamber. You would then use the decocker, with the gun pointed in a safe direction, to decock the weapon.
You would not have your finger on the trigger, or preferrably not even near it. What I am saying is that the tragedy of mistaken sequence of clip loading and accidental shooting as I described is not unknown, and has happened to two people I know - whereas a dual failure of both extractor and firing pin is unheard of. I asked a friend who works at gun shop with a shooting range about this this weekend, and he concurs that he has never, in the tens of thousands of customers on the range over decades, heard of the sort of failure you seem to propose. However, the sort of human failure I propose happens about once a month - including to police officers, who also use the same range (since many local PDs don’t have their own range). Thankfully, most of the time the gun was pointed downrange so no one was hurt. However, there are more than a few bullet holes in the ceiling and divots out of the floor resulting from times when the weapon was not safely pointed…
He was presumably trying to show how to check to make certain that the round had cleared the chamber. My opinion is that the round had in fact cleared the chamber, and by human error (perhaps being distracted by the demonstration) he put the clip back in, then worked the slide. If he had the gun held at an angle to show the innards to an audience member, it may very well be he was holding it improperly and had his finger resting on the trigger when he worked the slide shut. Holding my 9mm this morning in a test (what every girl should do on a Monday…), if I rest it to the side my finger does naturally lay across the trigger. If I turn my hand the other way, to the left, the trigger rests driectly on my finger unless I make an effort to curl it out of the way.
The “SNAP” of the slide does jerk the gun a bit on some large guns, and if his finger was just resting on the side of the trigger, it might have fired it.