Down by eight points with about nine minutes left, the San Antonio Spurs were looking to put together a few stops so their offense could get them back in the game.  One way was trying to trap Rajon Rondo in the backcourt, forcing the basketball out of his hands.  However, a poor angle taken by one of the men trapping Rondo lead to a fast break coming out of a timeout:

This starts with Glen Davis entering the basketball to Rajon Rondo along the opposite baseline.  As soon as Rondo gets the basketball, both Manu Ginobili and Tiago Splitter head towards Rondo, trying to trap him.

However, Splitter takes a bad angle on his way to Rondo, giving up the sideline.  Rondo instantly sees the open sideline and takes it, breaking the trap and heading up the court.

With Rondo getting the sideline, and with Davis heading down the court, the Celtics now have an instant 5 on 3 fast break as Rondo takes the ball towards the rim.

Eventually Gary Neal steps up, forcing Rondo to pick up his dribble, but Rondo, who is keeping his head up, spots Davis who is running along side of him.  He is ahead of Splitter, who is still trying to recover from taking that poor angle.

Davis makes the catch and is able to finish at the rim with nobody on the Spurs stepping up to him.  Here is the play in real time:

When you are taught how to trap in the backcourt, you are taught to first protect the sideline and then worry about trapping the ballhandler.  On this play, Splitter runs straight at Rondo and that is what allows Rondo to take the sideline.  So instead of getting a trap on Rondo and forcing the ball out of his hands, the Spurs basically give him a running head start into the frontcourt, allowing him to create a fast break opportunity where he gets an easy bucket in the fourth quarter.