Continuity mistake: When Linus is rolling the pumpkin on the path, the path is plain, but when we see Linus following Lucy in the next shot, the path is slated.
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.
Other mistake: When Lucy asks for an extra piece of candy for her brother during the Trick or Treat scene, her right hand has five fingers when she holds it up, but her left hand only has four fingers.
Answer: From what I've seen, usually Sally is often annoying Linus with her flirting when he has nothing in common with her, or he's just trying to do something to help her and she tries to make it into more. Here, Sally's comments make Linus think she believes in the Great Pumpkin too, or is at least interested in hearing more about it. It's more that he's excited she likes the same thing as he does. Later in the pumpkin patch, he's happy she's there because he wants to share the experience with someone, not because he has any feeling towards Sally.
Bishop73