Revealing mistake: At the Halloween Party, a girl with a green dress says to Charlie Brown, "Charlie Brown, you'll have to model for us." When she says this, she suddenly grows a third hand, then she has two hands again a little later.
Continuity mistake: When Linus and Lucy are heading to the pumpkin patch at the beginning of the film, Linus takes his blanket with him, but when they pick up a pumpkin and march back, Linus no longer has his blanket.
Suggested correction: When they get to the pumpkin patch, Linus drops his blanket. When they leave, he leaves his blanket at the pumpkin patch. He might have forgotten it, but it's not a mistake.