Oh yeah, I see this all the time. That's one of the reasons why I try to make sure I board early... to make sure my seat isn't raped and pillaged before I get there. I mean, aside from pillows and blankets, I've seen people take amenity kits, put their carry ons in the overheads, put their coats in the closet, headphones, anything that isn't bolted down...

It's probably why UA doesn't put out the amenity kits before boarding anymore, instead opting to hand them to you after you're seated.