This is interesting...
https://en.wikipedia.org/wiki/Perfect_season
The best start from an NFL team who failed to complete a perfect regular season is shared by two teams:
the 2009 Indianapolis Colts, who started 14–0 before losing their final two regular season games to the New York Jets and the Buffalo Bills to finish 14–2
the 2015 Carolina Panthers, who went 14–0 before losing to the Atlanta Falcons and going on to finish the regular season 15–1.
Four other teams have started 13–0 before losing their fourteenth game: the 1998 Denver Broncos, 2005 Indianapolis Colts, 2009 New Orleans Saints and 2011 Green Bay Packers.
We all know about the 16-0 Patriots finishing 1 game shy of the SB at 18-1
'84 49ers, '85 Bears and '07 Patriots tie for the most total wins (18) in a season through the SB.