The Secret of the Unicorn: Part 1 - S1-E3
Continuity mistake: Tintin runs up to Captain Haddock's apartment door and begins knocking on the door. When Tintin begins knocking, the colour of Haddock's door is brown. In the next shot, the colour of the door is suddenly red. Then in the shot after that, the colour of the door is suddenly back to brown again. (00:10:25)