Possibly the most successful summer replacement show in terms of the number of seasons was Hee Haw. It started life as a 13 episode order to replace The Smothers Brothers on CBS in the summer of 1969. I read that Roy Clark had reservations about the show hurting his career and his agent told him that no one would watch it! Hee Haw got the ax from CBS in 1971 during their infamous "rural purge" but it was picked up in first run syndication where it went until 1993!