Marge clearly states that Snowball II was bought, while in Lisa's Substitute (1991) from season two, it's stated that Snowball II was a kitten the original Snowball had.
When Marge tells Homer to close his eyes as she takes out the money jar, her necklace is red. When she takes it out of her hair in the next shot, the necklace is white.
At the racetrack, there are only 6 traps for the dogs to run from, yet there are eight dogs (Santa's Little Helper is number 8)
Marge comes through the door and you see that she has come from a room with a piano in it. It has a music book on it. But when the shot changes so we see Marge turn the jar upside down, the music book has disappeared.
When Millhouse, Bart and Lewis are talking about the fake Santa, Millhouse's eyebrows go from blue to black to blue again.
The copyright date is given as 1990, yet this episode aired in 1989.
When Marge is talking on the phone to her sister in the beginning, when she goes to hang up the cord completely disappears from the receiver
When Marge thanks Homer for Santa's Little Helper, during one of Homer's lines, the background behind Marge and Homer appears upside down.
When Skinner steps up to the podium after the first graders sing, he is not drawn to scale, appearing as tall as the children behind him.
When Bart and Homer return with Santa's Little Helper, a single frame of Maggie sliding off Marge's lap was misplaced.
When Homer walks out Circus of Values with his purchases, he bumps into Flanders, and all the presents scatter everywhere. Watch out for a red present with a green bow that goes flying near the left hand side of the screen. As soon as it hits the ground it turns into a pink box with a white bow wrapped around it, with no shot changes.
In Sherri and Terri's first appearance, their outfits are blue instead of purple.