I just discovered this. On a 2000 v6 Stang the fuel filler tube has a 6" piece of split-loom tubing zip tied on to it right before it enters the tank. It seems as this split-loom is the perfect condensation keeper, which has allowed the metal filler pipe to rust completly through and start leaking whenever I fill up with gas (and possibly as I'm driving, not sure).
I'm putting some fuel proof putty stuff on as a temporary fix. Planning to replace the whole tube once I can find one.
FYI
Might want to take a look under there before some guy next to you at the gas station says "Yo, dude. You car is leaking gas on the ground..."
-Scott