Back in the late 70's, I worked for Dixon Ford in Vista, CA.
A client 
brought in a vehicle which he said would not tune up,
and he'd taken 
it to half a dozen places.

I noticed a hiss from the exhaust when I 
tried to rev the engine
and that led me to the gas filler, whose small 
"Unleaded Only" diameter
adapter plate had been less than cleanly opened 
up. I was surprised
that he didn't blow himself up with the sparks from 
that chiseling.
The owner told me that he was on his third tank of 
leaded gas.

I tracked the problem down to the catalytic converter 
being plugged,
and once the new one was installed, I cut the old one 
open to see how
the plug had come about.

Attached is a picture of both 
good honeycomb catalytic element
and one piece from the front after the 
leaded gas. It got quite hot,
as you can see. I believe the metallic 
structure of the honeycomb stayed
intact, though.

On the left, the 
output section; on the right, the plugged input.
It appears that the 
fuel additives danced on top of the hot converter
until they coagulated 
and melted, plugging up the front of the honeycomb
and cooling it at 
that point.

About 3/4 of the input was plugged like this.

It took 3 
weeks to get a new filler neck for him, too. <g>