I am seeing those coming from high end lighting stores. I had one where I roughed in a 4" octagon then came back for the finish. They had purchased a bar fixture that was too skinny to cover the box, needed a one gang mounted sideways. There is no way that fixture weighed less than six pounds, it was four feet long with three lamps with heavy glass shades. I mounted it low on the octagon so the part that wasn't covered was at the top where it couldn't be seen behind the center shade, then made up a metal cover for the gap. I told them it was a non standard fixture and it wouldn't be safe or legal to hang from a one gang device box. It was made in England.
It seems like customer purchased lighting is always throws a wrench in the works.
Brian