I saw this happen repeatedly when I was flying to L.A. every week for 6 months.
This flight was always full, that is, there was little overhead baggage room. Yet, people would put BOTH of their bags above them and refuse to put anything under their seat. Now if wasn't a full flight I wouldn't have a problem with it, or if you only had 1 bag and put it over your head...but the attendant specifically asked everyone to put one bag under your seat. So now people with their allowed 2 bags, can only put one under their seat and have nowhere to out the other.
This didn't happen to me personally since I usually have enough elite status to board first, but I still felt like some people are just rude. I even noticed the same people doing this week after week.
I'll bet they don't even think they are doing anything wrong, so I am here to set the record straight: If you have ever done this YOU ARE INCONSIDERATE!