Ive had it snow on me on younger ac types! Especially departing hot n humid destnations! Many airlines dont use the apu anymore due to fuel burn. Therefore the ground conditioning carts are used that (imo) add more moisture as they are rather poor at coping with the large mass of ducting on big a/c.
When the packs are used again after start, or take off, that moisture , that should have been removed by the air cycle machine, freezes in the ducts and blows out down the side wall vents. On older aircraft ive seen the walls get washed, saves a cleaning bill i suppose.