All that happens is that the pressure in the tank will be a bit lower, but way over 12 psi. If it drops from 600 psi to 500 psi in the fridge it's not a problem. The gauge drops the pressure, if you set it to 12 psi and if the tank has something 12 psi or higher, you'll get 12 psi to your keg. It doesn't matter if the tank is 13 psi, 100 psi, or whatever.