is mental, not physical.  

He was the last one (besides Monta) to learn Nellie's defense, and when he did the warriors went on their run.  I don't have the quote but Nellie said something along the lines of "(Harrington) finally figured out your guy can't score if he doesn't get the ball."  Rotating to the ball plays a big role in Nelson's defense.

Then, on the break, Al missed Pietrus with his pass on a 3 on 2.  As Reggie pointed out, Pietrus had set up for a 3, which seems to be the status quo for the warriors on a break.

Monta, by the way, rotated to a guy who was already covered, allowing the offense to find his man, who then hit a 3.  Not once but twice.

Barnes, Monta, Pietrus, and Al are still making mental mistakes.  Time in Nelson's system will eventually fix this, and I think the warriors are only going to get better.