This slip up occurs during the scene where Ron, Harry, Hermione and Hagrid are at the lake and Hagrid is telling the kids about how Buckbeak is to be put to death. Hagrid is explaining to the children about the court's procedure and you see Harry, who is wearing a black/navy short sleeved shirt, start to walk toward the lake's shore. In the next scene, you see Harry in a long sleeved black/navy shirt sitting on a rock.