Other mistake: When Snoopy starts sneaking through the pumpkin patch, Sally is standing with her arms crossed and Sally's hand is painted blue while talking to Linus. (00:20:35)
Continuity mistake: When Linus mails his letter to the Great Pumpkin, it can be seen that the mailbox has no writing on it. On close-ups, "U.S. MAIL" is written on the side of the box. (00:07:20)
Continuity mistake: The fence which Linus tries to shove the pumpkin through at the start of the show, loses planks from one side to the next. On one side, the fence (to Linus's right) has nine planks. When the view changes to the opposite side, this same section of fence now has only seven planks. (00:01:05)
Other mistake: When the kids leave Linus standing alone in the pumpkin patch, Sally decides to rush back to Linus, and when Linus explains that she'll see the Great Pumpkin with her own eyes, Sally's left arm is blue, the same color as her dress, then instantly changes to the normal flesh color within the same shot. (00:11:20)
Continuity mistake: When Lucy awakens at 4:00 AM she checks on Linus and then puts him to bed. In some shots there's a window over Linus' bed, but in other shots the window has vanished, and there's only a shelf over the bed. (00:23:10)
Continuity mistake: When Linus and Lucy are walking to the mailbox, he has a letter in his hand. For a couple frames this letter and his arm suddenly disappear before returning.
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