As Long As We're Together (1) - S5-E1
Factual error: The start of season 5 took place in 1881 (the year Mary first went to the blind school). In 1881, Grace (born in 1877) would have been around 4 years old by then, but she is shown as a baby in the series.
Laura: I wanted to clean the blackboards extra good today, to make up for being late.
Mary Ingalls Kendall: And talking in class?
Laura: I was not talking.
Mary Ingalls Kendall: You were, too.
Laura: Was not! Nellie was talking and I was answering.
The Voice of Tinker Jones - S1-E11
Trivia: Sean Penn is in this episode, albeit with no lines, always wearing the same blue shirt. His father directed this episode.