Some Guy From Mars wrote:What about the 1966 Green Bay Packers?
The 1967 Packers were undefeated after four games--at 3-0-1--when they lost at home to the Vikings, 10-7. Similarly, both the Browns and Cardinals started the 1964 season with 3-0-1 record--that tie was a 33-33 classic between the two teams in Week 2. The Cards then lost at Baltimore in a then-rare Mnday night game, while the Browns had gotten ripped apart by John Henry Johnson two nights earlier.
1968 Giants beat four weak teams (Steelers, Eagles, Skins, Saints), then lost at Atlanta. After 10 games, they were just a game behind the Cowboys before losing their last four games.
1975 Cowboys went 4-0 and then somehow lost by two points to the 0-4 Packers--they ended up reaching the Super Bowl. That same year, Buffalo was obviously looking past a lousy Giants team with the Dolphins next up. Giants beat them 17-14, with the Bills dropping six of their last 10 games.
1976 Rams were another "undefeated" team with a 3-0-1 record when they got sacked by the Niners on MNF, 16-0.
1977 Raiders started out 4-0 and then discovered the Broncos were for real, losing 30-7--the first of three games between them that season.
1979 Dolphins started out 4-0 before losing to the Jets at Shea, 33-27. They finished 10-6 and were then knocked out in the playoffs by the Steelers.
1980 Chargers lost to the Bills at home in Week 5, 26-24. The two teams would meet again in San Diego during the playoffs.
1981 Cowboys lost to the Cardinals, 20-17, then went on the NFC Championship game. Meanwhile, Miami was 4-0 and tied the Jets.