When the fueler asks how much you want in your tanks, if you accidentally/purposely click "Give me this many gallons/liters" without moving the slider (that is, the number set is still zero), he will go out and pump until the valves close, as if you had told him to fill the tanks.
I tested until the mains and aux tanks were full, and stopped him when I was down to just the tail tank so as not to spend 15 more minutes waiting.
Granted, this is better than a divide-by-zero crash, for sure - but I'd prefer if he asked me if I was sure I wanted zero gallons!