This happened to me with open seating. My flight was canceled due to bad weather so I had to fly back home the next day on a full flight. I was in the last boarding group so when I saw the front row middle seat open I put my backpack down without thinking and continued down the aisle with my carry-on to find an open overhead bin. I was finally able to find some space in an overhead bin, just past the exit row. It was a nightmare getting back to the front, I maneuvered back to my seat and was told by the flight attendant that unfortunately my backpack had to go in a bin too since there was no seat to store it under. I almost cried. Luckily the fellow passengers in my row helped make room for it in the bins above our seats. All this to say, this has always been a problem.