Factual error: Adam Kendall goes to Minneapolis to take the entrance exam to law school. He meets a fellow student who, after the first exam, accompanies him to the "EL (the elevated train) " Minneapolis never had an elevated train, only Chicago.
The Nephews - S7-E14
Factual error: Royal Wilder promises to give his sons an Indian Head nickel if they are well behaved at Almanzo's/Laura's house. The Indian Head nickel was not minted until 1913 - 30 to 40 years later.