I once made the mistake of thinking that the bag in my space had been left on from the previous flight. The passenger wasn't there at the time, having wandered off to the toilet or gone exploring or something. I handed it to the FA saying something like, "This was in my space, I think it must have been left on from the last flight." The FA took my statement at face value and off the plane it went. It wasn't till we were halfway to SYD that the passenger in front turned around and came looking for his bag. There was quite a scene...