in CA ( and maybe other states) the new nozzles at the gas stations are designed to collect all the vapors coming out of your tank as it fills with gas. This means that you can't back the nozzle out and still pump in gas. There needs to be a tight fit to the fill spout and the nozzle's plastic hose piece.
It's weird, because I CAN pump in gasoline in my motorcycle's tank while I watch it get filled and see the gas in there. It ALSO shuts off too soon when I insert the nozzle all the way! LOL! You can't do that with a car though.

Dealerships won't be able to replicate this, so you might want to try to fill up at a station near the dealer, and if it doesn't work there offer to drive the service rep there to show him how you can't fill up your Mustang! This issue is under TSB, but all service techs seem to be oblivious, so find the TSBs, search MF for fixes that have been done and show him all the proof.

Just get it done while you are still under warranty.