Bleed air + ground air
Airbus states that APU bleed should not be on with ground air plugged in (in other words providing conditioned air at the same time) due to damage of certain valves? But when was the last time you arrived on stand to have ground ops plug in air and actually advise you that they've done so? (normally you get the buzzer or a knock for ground power). Been flying commercially 7 years and never seen this happen. The one doing the walk around normally discovers it and then advises the one in the cockpit to deselect APU bleed.
We can't make the assumption that if ext power is connected so will ground air be connected as that really depends on where you're going and what's working/ being provided on that day by handling. Does your airline have a policy for determining / being advised that ground air is plugged in?