This was originally planned as an episode for the third season but was moved up when the writers decided they wanted to do a flashback episode that involved some of the show's supporting characters.
At the end of the episode, when Bernard gives up on building the big S.O.S sign on the beach, he has gathered two unused piles of rocks, and assembled the upper curve of the first S. If someone were to see this incomplete message from a plane or a satellite as he hoped, it would look like a frowny face.