The sequence happened like this. The Redskins are driving toward a game tying touchdown. RG3 completes a pass very close to a first down marker. It looks to be a little bit short, so Mike Shanahan asks for a measurement and this is what the ref allegedly told him.
Shanahan on that last sequence: "Well, I said I wanted a measurement and he said, 'You don't have to, it's a first down.'"
— Chris Burke (@ChrisBurkeNFL) December 2, 2013
As you can see from the photo, the down marker has changed to 1st down and the chains have moved. Thinking it is 1st down the Redskins call for a deep pass down the middle.
Fred Davis drops the pass and to Shanahan’s surprise he is now being told it is 4th down not 2nd down. Because the Redskins need to make sure they pick up the yard, they call for a very short route to Pierre Garcon (it is highly unlikely they send their best receiver in the flat if it was 2nd and 10).
Garcon catches the pass, but is stripped and game over.
The confusion led to play calls that necessarily wouldn’t have been called if the ref in this case Jeff Triplette (who has a history of screw ups) would have just taken control of the situation.
It doesn’t excuse the drop passes on the drive or Garcon getting stripped, but it was a serious error on the refs part and they need to be held accountable.