Not a safety issue, but a related item. On a flight the other day I saw a flight attendant make people remove duffle bags that could fit under a seat so that someone else with a bag that couldn't could have the spot.

If it had been my bag that was involuntarily shoved under my seat when I was only taking one overhead bin space I would have been seriously PO'ed. First come first serve on one bin space per pax should be the rule. I check a bag specifically so I can have my leg room clear and only take up one spot in the overhead for my large briefcase.

If you board too late and want to bring a large bag onboard that can only fit in an overhead and there are no overhead spaces left then it should be gate check for you. No exceptions. It shouldn't be my problem that pax Jones insists on bringing a huge carryon aboard.