Revealing mistake: When Whiplash is in the cave and Dudley comes and checks it out, the cave falls in. You can see a black curtain fall down while the screen goes black.
Continuity mistake: When Dudley's hat is burning he pours water on it. After the fire is extinguished Dudley looks up and you can see that the hat is not burned. After Nell sings her song you can see a large burned portion of the hat gone.
Continuity mistake: When Dudley extinguishes the fire on his hat, he doesn't get very wet. Seconds later when he opens the front door of the cabin, he is drenched. After he closes the door, he is completely dry.