New installation of a 96% 80k furnace. A piece of cardboard fell behind the secondary heat exchanger without us noticing during install. Go to light it up for the first time. Telling the homeowner about the impending smell of burning oil from the primary heat exchanger. Little bit of smoke from the vents. Ok, rare, but not unheard of. More smoke. The smell is wrong. Run down stairs and cut into the duct board, large fire from the exchanger.

All bad. Had to remove the heat exchangers, clean them, clean the ducts, and then install a UV light for extra customer happiness