In the last fifteen minutes of the film, Sofi sees a young Howl in a flashback scene. In this scene, Howl's hair is blue/black, but as we know from earlier, Howl gets his blue/black hair half way through the film when it is accidentally dyed. Young Howl's hair should be blonde like it was when we first meet him in the opening scene, yet it is not.
When Sophie leaves her bedroom, her dress has turned from green to blue. However, she couldn't have changed dresses because none of them would fit her after she was transformed (wider, much shorter, etc.).