I remember one flight where I was in the window seat of that row, with the empty space in front of me. I went to store my luggage under the seat two rows in front of me, only to find that the pax in the middle seat in the row in front had already put his stuff there so that he would have more leg room.

Fortunately, he moved his bag without much discussion, but I'm not particularly fond of this kind of "storage poaching". Maybe he was just oblivious, but I still find it annoying.