I've been on flights where people have placed their bags in flat and sidewise then just closed the bin. Attendants then get upset when you open bins to check for space (even in cases where you saw there was space before someone closed it) because of course they are the only ones that close bins and they always check for space.

I've also been on flights where people have stood up to use the bathroom or adjust their bags while taxiing and then everyone has to wait on them wine the attendants just repeat on loop that they need to sit the fuck down.

The rules of planes just seem not to matter at all to some people and the rest of us have to suffer for it.

I was on a flight earlier this year where the attendant had to explain repeatedly to the same group of passengers that they may not put their winter coats into the overhead bin and will have to keep it with them. This same flight they had to restart the safety demonstration because someone got up and went to the bathroom during the first playthrough of it

All too often it’s an announcement like “boarding groups 7 and 8 must check their carryons”

What if you have stuff in your backpack that is not allowed in hold luggage (lithium batteries)?

Now we know why they keep running out of overhead bin space

It was a flat road case for an electric guitar, on a jumbo jet. There was plenty of space for 3 or 4 carry-ons or backpacks to sit on top of it and the bin in question was not even remotely full at take off.

Because musical instruments are specifically allowed by federal DOT rules?

Specifically: ”A small instrument must be accepted as cabin baggage when it fits safely in an approved overhead or under-seat space and that space is available when you board. Ordinary carry-on size limits aren't a reason to reject an instrument that meets those storage conditions."

https://www.airtravelquestions.com/tsa-rules/can-you-bring-a-musical-instrument-on-a-plane

And as I said in another comment, laid flat it fit quite easily in the overhead compartment with plenty of room for additional bags. The compartment over my seat was not filled before takeoff and was fully empty when I boarded. It would have been both fine and legal for me to stow it there, but the flight attendants insisted on gate checking it regardless.

Plenty of bags could have fit on top of my flat, 4-inch thick electric guitar case.

Oh! I carry a giant Camelbak, so it does double duty I guess.