A few years ago flying Qantas business class to Tokyo (overnight) the entertainment system failed. (So did all of the power seat mechanisms initially, but the crew fixed that promptly.) Until shut-eye time I had a very pleasant chat with an interesting neighbour instead, so I wasn't too fussed & didn't complain too strenuously. But I did receive a $200 voucher a few weeks later. For a flight where I planned to be asleep for much of the time anyhow, that seemed appropriate and I appreciated the unsolicited gesture.