On a recent experience with OZ long-haul F (A380 JFK-ICN,) I had misplaced my headset and thought maybe somehow it had fallen behind the seat back. Turned out it hadn't, but OMG, the amount of dust and debris back there! It wasn't just ONE cleaning that was skipped, it looked like it had been a looong time since it was cleaned. Bits of paper, food and even some cutlery was back there. Yuck.