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.
Welcome To Olesonville - S9-E3
Continuity mistake: Mrs Olsen cleans up the old Hansen house to sell, however it was made the blind school years back and burned down, killing Mary's baby and Alice Garvey.
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.
Trivia: Star Michael Landon was born 100 years after the real Charles Ingalls. Ingalls born 1836 and Landon born 1936. Lindsay/Sidney Greenbush (who played Carrie) was born 100 years after the real Carrie. Carrie in 1870 and the Greenbush twins in 1970.