It happened to me this past Monday on a UA flight from DEN-SFO. I arrived at my seat to find that my seat neighbor had his laptop bag protruding well into my legroom space. As I settled into my seat, I thought he would move it. He didn't. I waited until the plane started to push back, and I politely said to him: "Excuse me, could you slide your bag over a little bit so that I can occupy my leg space". By his reaction, I could tell he was unhappy that he had to move his bag. I made sure he moved it completely off of my side.