When the kids are going door to door and getting their bags filled, you can actually see the rocks being tossed into Charlie Brown's bag.
When Lucy goes bobbing for apples and comes up with one that has Snoopy on the other side and freaks out.
Lucy: Blecch! My lips touched dog lips!
When Schroeder plays "It's a Long Way to Tipperary", "There's a Long Long Trail" and "Pack Up Your Troubles" in succession, and Snoopy's responding mood shifts from happy to sad accordingly...
Culminating when Schroeder's rendition of "Roses of Picardy" provokes an "OwwwOOOOOOOoooooooOOOOhhhhhhhhh!" from Snoopy. note This scene was omitted from the 2015 ABC broadcast.
Snoopy rising out of the pumpkin patch and causing Linus to faint, then disappearing immediately after.
When the Great Pumpkin fails to appear, Sally gives Linus an epic dressing-down that culminates in her violently shaking him while hollering, "YOU OWE ME RESTITUTION!"
Charlie Brown trying to console Linus:
Charlie Brown: Well, don't take it too hard, Linus. I've done a lot of stupid things in my life, too!
He had inadvertently pushed Linus' Berserk Button when he said that. Linus proceeds to berate him all through the end credits.
Violet forcing Charlie Brown to be a model.
...for a jack-o'-lantern.
A brief visual gag with Lucy holding a copy of TV Guide with her own picture on it.
When Snoopy first sees Linus writing to the Great Pumpkin, the beagle starts laughing hysterically. Still laughing, he walks over to Lucy, who is holding the aforementioned magazine...and she promptly tosses him away like a rag doll.
Right after the above occurs, Lucy walks up to Linus, glances in annoyance at the letter he's composing, then abruptly grabs and shakes her brother violently.
Lucy: NOT AGAIN! Writing a letter to a stupid pumpkin?!
In the beginning, Linus, licking a huge sucker and is sticky as all get out, jumps in Charlie Brown's leaf pile and ends up with a leaf-covered confection and a "beard" of leaves. Quoth the leaf-bearded Van Pelt boy, "Never jump into a leaf pile with a wet sucker!"
This promo for a 2003 Canadian airing of the special.
Lucy asking Charlie Brown to kick the football, holding a signed document promising that she won't pull it away. Charlie Brown runs up to kick the ball, convinced that Lucy won't pull it away. She pulls it away, and Charlie falls flat on his back.
Lucy: Peculiar thing about this document. It was never notarized.
This webcomic gives a pretty funny alternative explanation for why the Great Pumpkin never showed up.