There were many momentum changing plays for the St. Louis Rams on Sunday. In my opinion the biggest play in the game was the Juqua Parker fumble return for a TD. After that TD the Eagles held onto the lead for good, and more importantly the Rams were never able to fully regain the support of the home crowd. The air was taken out of the Dome on that play.
Let's break down what happened on the play. Here's a link for a video of it from NFL.com. First off there Sam Bradford just lost his footing. Whether this was just a freak accident or because an Eagle's defensive lineman Darryl Tapp was in his way is unknown to me. When Sam fell over, the aforementioned Tapp took the opportunity to hit the ball out of Sam's hands. It was a great effort play on his part. After the fumble Tapp also was smart enough to hold Sam's foot so that he couldn't recover the fumble. This allowed Parker to scoop up the ball and run it 56 yards back for a TD.
Now players will slip at times and it is understandable, but to have a TD scored on you because of it is very tough to swallow. This was the momentum turning point in the game. Before the fumble the Rams had momentum on their side. They did just give up a TD and the score was tied, but the team was looking good and playing well. After the fumble the team, especially the offense never could get into a groove. The Parker fumble return for a TD wasn't the reason the Rams lost but it was the starting point and for that it was the most important play in the game.