Jason is noticeably absent from the family "good nights" at the end.
When Grandpa and John Boy go out to cut the Christmas tree, what happened to the snowstorm? The sun is shining and the sky is blue.
At the end of the movie, as John Boy runs from the kitchen to the front door after hearing his father approaching, there is a brief shot in which he is not wearing the jacket and scarf seen in all the other surrounding shots.
The calendar in the kitchen clearly shows Christmas Eve on a Monday, but in 1933 (which is when the story is supposedly taking place) Christmas Eve was on a Sunday.
Mary Ellen says the wrong title for a book of the Bible. It is the Song of Solomon and not Solomon's Song.
When the children are unpacking the Christmas ornaments, Jim-Bob jostles Elizabeth's arm and the ornament shatters. Erin can be seen mouthing Elizabeth's line: "You bumped my arm!" There are other instances of the children mouthing one another's lines.
After the sleigh turns back from the log in the road, a plastic blaze-orange 'Keep Out' sign in clearly visible in the background.
When John Boy, Hawthorne, and the Baldwin sisters set off in the sleigh, it's supposed to be the middle of the night. Yet the scene was clearly shot during the day.
When Charley brings in the turkey, he says he shot it and they ask how he did it and comment on how clean it looks. It's obvious immediately that it came from a store: it was clearly already plucked, gutted and wrapped in paper.
When John Boy enters Hawthorne's church, the last pew is empty. When the camera looks away, then back, someone is sitting in that pew. The person keeps appearing and disappearing repeatedly from camera shot to camera shot.
When Claudie reveals that missionaries are handing out toys, the kids jump up and knock Grandpa over in the rocking chair--listen carefully and hear the children laughing about it.
Miss Emily goes to play the old Victrola she says it hasn't been played since their father died, but a record is sitting right there ready to be played.
In one scene, the Waltons listen to "The Johnson Wax program": "Fibber McGee and Molly" on the radio. However, the film takes place in 1933 and that program did not debut until 1935--and the specific episode they're listening to is from 1947.
The Waltons lived in rural Virginia and it is highly unlikely that their house would have electricity in the early 1930s before the Rural Electrification Act. Even more implausible is the family having electric Christmas-tree lights, or electricity in their barn.
The electric train set given to Ben Walton for Christmas in the last scene is a Marx #54732 Pioneer set, which was not manufactured until some years after WWII ended. The Homecoming is set in the 1930s, so the train set is anachronistic.
The Zenith table radio is shown with a dummy Magic Eye indicator tube- probably just a light bulb behind a piece of frosted glass or plastic. While it's true that most of the Magic Eyes (like the 6U5) did glow green, the correct Eye tube for this model is a 6T8, which had a reddish tint to its display.
John-Boy tells Charlie Snead that his father used to let him drive "that old DeSoto he used to have". Chrysler started selling DeSotos in 1929, and this story takes place during the Depression, so there weren't any "old" DeSotos yet--and they didn't even own theirs anymore.
A Christmas ornament shatters on the floor and is ignored, even by the strict mother who would have assuredly ordered it cleaned up.