Flames leaving the safety of the heat exchanger cause the metal and component discolouration, in this natural gas furnace. It is commonly called flame rollout and normally occurs when the furnace is igniting its burner.

Damage of this nature is usually only visible after removing one of the furnaces access panels.

Flame role out is a dangerous condition, which can damage the controls of the furnace not to mention a very real fire hazard and the possibility of carbon monoxide poisoning.