Originally Posted by ajamieson

Happens sometimes, but usually only when there's an issue once airborne eg the tanks run out or the systems fail.

Would be odd to depart with such an issue, even on a short hop.

Once sat at the gate on a Delta 757 waiting to fly LGW-CVG while the ground bought bottled water to pour into the toilet tanks because the handling agent's mobile tank had been left parked out overnight and had frozen solid.