Other mistake: At the very end, Mr. Hanson is seen walking toward the church. Because of his stroke, he's slow and unsteady. We see several close-ups of his feet, showing how he's struggling. Two of those shots are the same footage. It's obvious, because you see that he drags his foot over a small stick in the road and stumbles - and in both shots, it is *exactly* the same.
Continuity mistake: At the end, when Lars Hanson hears the church bell ringing, he gets up from his bed and goes to look out the window. When he first gets up, his hair is completely tousled - definite "bed head." The shot changes to show him leaning out the window, and his hair is neatly combed. When the shot returns to the inside of the room, his hair is once again rumpled and untidy.
Revealing mistake: When Charles Ingalls plays his fiddle at the end of the episode, one can see from the movement of his hands that he is not really playing, but just pretending. (Also visible in every other episode where Charles plays the fiddle, but most obvious in this one.).