I think everybody knows rushing for a first down on third and 31 play is uncommon. How uncommon, though?
I went to the Pro Football Reference Play Index to find out.
After searching for all running plays on third and 31 or longer for over two decades, I found the answer.
On October 31, 1999, Leroy Hoard of the Vikings broke a 53 yard run on third and 37 in a game against the Denver Broncos. Pro Football Reference has a database of all NFL plays since 1994. That is the only other time it has happened in that span.
(As a side note, last year a Green Bay 8 yard run on a third and 35 play resulted in a first down because of a personal foul penalty. The opponent that committed the penalty was ironically the San Francisco 49ers.)
So if you were watching yesterday you can at least say you saw something happen for the first time in the NFL since the turn of the century.