A person I know told me he came across this scenario. If you get to your seat on a plane, the first thing you do is open the locker above your seat and put your case in, but suppose there is no room for your case because of people putting "other things" in, what do you do?
If I come across that situation, I will take the "other things" out and leave them in my seat then put my case in.
I will then wait until I see a member of the cabin crew and tell them about the "other things". Well the cabin crew member is not going to tell me to take my case back out and let the owner of the "other things" put them back in again as that means they are going to have to find a place for me to put my case. What would you do?