I've seen this a couple times this winter, kind of wondering if anyone else has also?
BTW, I didn't condemn this particular furnace, I only uninstalled it
This is from a 1997 Tempstar mid-efficiency. Though this side is worse, it's actually cracked on both sides of the cell:
I did kind of an autopsy on this puppy back at the shop. In my free time of course
With the burner rack gone
Looking into the center cell
That spiraly rod (NOx rod?), the end of it is touching the sidewall of the heat exchanger. I think it was guiding the fire and / or transmitting the heat directly onto that area.