This episode was the beginning of the end for Radames Pera (John, Jr.). According to Alison Arngrim (Nellie Oleson), "Michael [Landon] had decided to postpone the whole plot line about her [Mary's] blindness indefinitely and let her become a romantic lead." Pera's character, she said, was supposed to marry the eldest Ingalls' daughter. The problem, the Nellie Oleson actor said, was that "Melissa Sue hated his guts, refused to kiss him, and eventually he left the show." What that meant for Anderson, then, was that "Michael had to go back to the old plot line and make her blind."
On the DVD, this this is Episode 2 of Season 3, not Episode 12.