It's Magic, Charlie Brown (1981)

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 Charlie Brown is going to feed Snoopy, you can see at the far of the end counter there is a tin opener which is pointing to the right and there is nothing on it. But when he takes some dog food out of the cupboard, we return to the tin opener and it is now facing left with a plug attached.

Continuity mistake: When Snoopy is mixing earth with water in one of the buckets, the hole that he has dug the earth from changes between shots.

Lucy van Pelt: Linus, have you seen Charlie Brown?
Linus van Pelt: Nobody has seen him since Snoopy's magic show. I just talked to Sally, and she said even when you look at him, you don't see him.
Lucy van Pelt: What's that supposed to mean? That's the dumbest thing I ever heard.

Lucy van Pelt: All right, beagle, here's your magic book. You made Charlie Brown disappear. You'd better make him reappear. Now you start figuring out how to make him visible again, or I'll pound you.