I was in FC couple of years ago on a CO flight from SFO-IAH, stowed my bag in the overhead and then got engrossed in a book waiting for departure. A few minutes later, I saw my bag in the aisle by my feet. I had not even noticed when it happened, but someone across the aisle took my bag out so they could stow theirs!

An FA saw it at the same time I did and convinced the offending party to stow one of his bags under the seat in front of him and my bag made it back into the locker overhead. He was a bit perturbed about losing legroom....