Happened to me many years ago on a viscount out of jersey on the easterly runway.I was dead heading in a full aircraft on a jump seat in the cabin.I was forward facing at the rear and realised we were lining up to roll when I slipped the belts and went up to the flight deck as the throttles were coming up.No config warning fitted for that.
The captain thanked me back at the crew room but I still have doubts as to whether we would have made it.If I had been facing the other way then I would not have seen the trailing edge.The viscount was blessed with big windows,fate and all that.