Remember when Charles said that PB&J was Mary’s favorite sandwich (Ep. 2 Season 4)? Well, Mary was born in January 10, 1865. And peanut butter was invented in 1884. Which means Mary would be 19 and already blind by the time peanut butter was invented. She would have been eating bread and butter sandwiches then.
When Laura told the story of putting apples in her dress, she said the boy she had a crush on was named Jason. While she did indeed once have a crush on two boys named Jason, the boy in the episode mentioned was actually named Jimmy Hill.
When Laura told Mary that she saw Almanzo with a saloon girl, she gave Mary an incorrect description of the saloon girl.