What is the longest down in NFL history?

Answer 1

The longest down in NFL history was in the game between the Detroit Lions and the Chicago Bears on Sept. 17, 2006. After jumping offsides 3 plays in a row, the lions were then penalized 4 times consecutively for holding resulting in an amazing 2nd down and 65.

They were unable to convert.

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This is inaccurate.

First, it's impossible for the offense to commit an offsides penalty. They would have to false start to do so, at which point the play is dead.

Second, the longest down and distance in that game, according to NFL.com, was 2nd and 31.

Answers.com won't allow me to link the source, but go to gamecenter on NFL's site and look up that game.