It's repeatedly stated in this episode that Mr. Hanson founded Walnut Grove fifty years before. Yet the sign at the end of the episode states that he did so in 1840. It was stated earlier in the season ("The Winoka Warriors") that the date then was November, 1880, and this episode takes place in early July of the next year.
At 1:11:39 as everyone is walking into the church singing Joy To The World, Andrew Garvey trips and falls into the backside of Alice Garvey.
The harmonica player at 9 min mark is not playing the tune heard.
As the wagons begin to leave the town, you can see some of the blind children behind Adam and Mary break character and begin to look around.