The Cardinals had Yadier Molina and Allen Craig in scoring position with one out and Jon Jay at the plate. Jay hit a groundball right at Dustin Pedroia, who made a diving play to stop the ball. Pedroia fired the ball to Jarrod Saltalamacchia, who tagged out Molina for the second out of the inning.
Saltalamacchia fired the ball to third base attempting to throw out Craig. The ball skipped past Will Middlebrooks, who made a diving attempt to stop the ball. Craig, who missed the Cardinals first two post-season series with a sprained foot, started to run towards home plate but tripped over Middlebrooks.
Third base umpire Jim Joyce called obstruction on Middlebrooks, which is clearly defined in the rulebook:
OBSTRUCTION is the act of a fielder who, while not in possession of the ball and not in the act of fielding the ball, impedes the progress of any runner.
The rulebook also includes a commentary on what counts as in the act of fielding the ball, outlining the exact situation that Middlbrooks found himself in:
If a fielder is about to receive a thrown ball and if the ball is in flight directly toward and near enough to the fielder so he must occupy his position to receive the ball he may be considered “in the act of fielding a ball.” It is entirely up to the judgment of the umpire as to whether a fielder is in the act of fielding a ball. After a fielder has made an attempt to field a ball and missed, he can no longer be in the “act of fielding” the ball. For example: If an infielder dives at a ground ball and the ball passes him and he continues to lie on the ground and delays the progress of the runner, he very likely has obstructed the runner.
Joyce ruled that Middlebrooks was no longer in the act of fielding the ball, and by continuing to lie on the ground, he delayed the progress of the runner. World Series crew chief John Hirshbeck clarified after the game that "there does not have to be intent" to call obstruction, so it does not matter if Middlebrooks intentionally attempted to slow down Craig.
The call invoked rule 7.06, which reads as follows:
When obstruction occurs, the umpire shall call or signal "Obstruction." If a play is being made on the obstructed runner, or if the batterrunner is obstructed before he touches first base, the ball is dead and all runners shall advance, without liability to be put out, to the bases they would have reached, in the umpire’s judgment, if there had been no obstruction. The obstructed runner shall be awarded at least one base beyond the base he had last legally touched before the obstruction. Any preceding runners, forced to advance by the award of bases as the penalty for obstruction, shall advance without liability to be put out.
Craig raced towards home plate, but the throw beat him to the bag, and Saltalamacchia tagged him out. The home plate umpire, however, ruled that Craig would have scored if not being obstructed, and awarded Craig with home plate.
If Craig had stopped he may not have been awarded home plate, thanks to rule 7.06 (b):
(b) If no play is being made on the obstructed runner, the play shall proceed until no further action is possible. The umpire shall then call “Time” and impose such penalties, if any, as in his judgment will nullify the act of obstruction.
As it stands, Craig scored the walk-off run on an obstruction call, giving the Cardinals the Game 3 victory and a 2-1 lead in the World Series.