The Seahawks have been dominant on third-and-short running plays this season, but they faltered Sunday in Arizona. Rookie LG Rob Sims made an assignment error on the third-and-1 play that gained nothing. This was the first time all season that a third-and-1 running play did not get the first down for Seattle. The team also lost yardage on a third-and-2 play to the right side (again, the first time all season a third-and-2 running play did not work for Seattle). These were both run from regular personnel (2RB,2WR,1TE). There was also a third-and-2 touchdown for Seattle, but that one was on the pass play to Darrell Jackson.
One blown assignment can turn a perfectly conceived play into a disaster. It happens. At San Francisco a few weeks ago, one of the Seahawks' best players, Walter Jones, simply missed his guy on a critical fourth-and-1 play. If Jones makes his block the way he does 99.9999 percent of the time, it's a good play. Against Minnesota a while earlier, Mack Strong missed his assignment on another fourth-and-short run that went nowhere. Air-tight execution is critical in those situations because drives are on the line. That is very easy to say from the press box, but much harder to execute when 300-pound guys are flying into your face.
The chart below breaks down every third-down play by distance needed for a first down: