Corrected entry: Jason mentions to Laura that someday there will be a "steam powered airplane." The word "airplane" did not even exist at that time - "flying machine" or so would likely be used during that era.
The Campout - S2-E5
Corrected entry: Very obvious that stunt doubles are going down the rapids on the branch instead of Nelly and Laura.
Correction: According to Alison's autobiography (Nellie Oleson) they did the stunt, not stunt doubles.
Correction: The WORD "airplane" already exist on that time, from 2nd half of 19th century, in french "aeroplane", check on pls.