classical,
The Fel-Pro Print-O-Seal gaskets had no shim steel reinforcement, they therefore got the nickname "Fail-Pro Print-O-Leak."
The problem arises after a number of heat cycles on a street engine some parts of the gasket creep away, creating an opening between the intake and the head. If you were lucky, it was a vacuum leak; if you were not, it was a coolant leak into the engine. Race engines didn't have that problem because their intake gaskets were replaced regularly - not so for the street engines.
Here's a thread describing and picturing one such failure:
390 problems | Capital Area Cobra Club
We have experienced these failures on all kinds of common Cobra powerplants including FEs, 385s, 289s, 351s, you name it.
After we cornered the Fel-Pro tech guy at a PRI show some years ago, they switched to a steel shim reinforced design which seems to have cured that problem. I understand the steel reinforced gasket has a "S" suffix to identify it. I still prefer the Victor Reinz...
Tom