When Milhouse dares Bart to pull Santa's beard off, Lewis changes from black to yellow.
When Milhouse, Bart and Lewis are talking about the fake Santa, Milhouse's eyebrows go from blue to black to blue again.
When Marge puts the phone down the phone wire disappears. This is more easily seeable in the DVD where you can slow-mo.
While Marge is writing her Christmas letter, the flames from the fireplace appear over the family's stockings.
Creator's Favorite Episode: In 2014, it was selected by current showrunner, Al Jean, as one of five essential episodes in the show's history.
First Appearance: Every supporting character aside from Grandpa. That includes Ned and Todd Flanders, Patty and Selma Bouvier, Mr. Burns, Moe, Barney, Principal Skinner, Ralph Wiggum, Sherri and Terri, Dewey Largo, Snowball II, and Santa's Little Helper.
This is also Milhouse's first appearance in the series proper (his actual first appearance was in the first Butterfinger commercial).
Also, Smithers is heard (but not seen) speaking over an intercom.