It's typically very cold outside the airplane. Where condensation forms and touches a freezing surface, it may also freeze, and there's ice inside the airplane. I've seen ice or frost in the airplane many times. Probably not a big deal.
I spent about a year and a half flying a corporate piaggio around. The door seals in those airplanes seem to be famous for leaking. A common trick used there and in other airplanes was to pour a cup of water slowly into the parting line between the door halves. As it was drawn out through the air leaks by the door seal, it would freeze. We would know about the leaky seal because of the whisteling sound it made, and we'd know when the ice had stopped it because the whistle went away. It only took a few seconds to freeze and plug the leak, worked like a charm, every time.