At a recent seminar, the instructor told of a rental car that he destroyed the cat in a hundred miles drive. I can't remember what was wrong with the auto but the cat died because it got overheated from too much fuel.
The modern car is designed to put the exact amounts of fuel and air into the cat and it makes the continued conversion to water, carbon dioxide, and heat. Anything that causes the proportion of fuel to rise causes overheating in the cat. The internal structure of the cat is basically a ceramic and will take more heat than most metals but as it is right in the fire they melt into a ball of glass fairly quickly. This is the reason for the strict misfire counter rules in OBDII. The quickest way to get too much fuel is to have a misfire where the whole charge of fuel winds up in the exhaust (such as a plug wire arcing). You can immagine the energy the cat must dispense with.
Bosch Master, ASE Master, L1
33 years MB technician