Sherman 1976 (10-0-2) and 1977 (9-0-1)
This is from lonestarfootball.net '76 Sherman/Temple game.
"Very painful game for long-suffering Bearcat fans, who hadn't enjoyed the playoffs since 1962. After being ranked #1 all season, Sherman let Temple score 2 TDs in final minutes and advance on penetrations, where they ultimately lost to SA Churchill in state championship game. The following season, Plano tied Sherman in district play and advanced to the playoff based on first downs, ultimately beating PN-G in state finals before what was at the time largest crowd in state championship game history. Again, Bearcats had been ranked #1 most of the season. This was before multiple teams advanced. Bearcats are the only team in modern history to go undefeated two straight seasons and yet not win a state title."