I was on a transoceanic flight, and the lady behind me kept knocking her knees into my chair. I looked behind me, and she's not only like 6'2", but she has her knees directly in front of her so as to present the greatest obstacle possible. No, can't put your knees in any other position, that would be too easy. We have to recline our seat while preventing the person in front of us from doing the same with his. I repeat: this lady had her knees out in front of her in the most obstructive possible position when she could have chosen to put them any other way. She knew damn well she was hitting the back of my chair.
It's people problem. Inconsiderate assholes who can't put themselves in other peoples' shoes - and get really self-righteous about it.