Continuity mistake: The buttons on Amos Slade's shirt repeatedly change sides between shots throughout the movie. One example is while Copper and Tod are swimming, when Amos tells Chief that Copper has strayed off again, the buttons on Slade's shirt go from being on the left to being on the right, then back to the left again. (00:21:50)
Continuity mistake: When Snoopy is at the library, he picks out a book and you can see there is nothing on the front cover, but when we see a close up of the book in the next shot, the words "How to Perform Magic" have appeared.
Continuity mistake: When Yosemite Sam tells the devil that he has "the very varmint in mind", you can see there are no flames coming up from the pit behind him, but in the next shot flames have suddenly appeared.