The Clint Longley game in 1974 pales in comparison to the 1979 season finale where we blew a 13 point lead with under 5 minutes left to not only knock us out of the division title, but of the entire playoffs. I defy anyone to cite a more devastating loss in your fandom.
Of course at the end of the game the refs didn't allow us a timeout which would've given Moseley a chance at a 60 yard FG for the win. I was 12 and out of the DC area at a time, but what I just discovered last week was that Moseley actually attempted two FG from over 70 yards that season (of course missed them, as the record Justin Tucker set a few seasons ago is 66).