I saw somebody on a recent flight board with a regular rollerboard carry-on as well as an enourmous North Face duffle bag that had straps so it could be worn as a backpack. Neither of the bags would be anywhere close to fitting under the seat, and it was definitely well past the normal defintion of a backpack as a personal item but neither the GA or FA even seemed to notice. I've also seen the trend of people taking really big softsided bags that are way bigger than the allowed dimensions for a carry-on, but sadly nobody seems to question the size of non-rollerboard carry-on bags.