The importance of knowing the situation
With about 10 seconds left in the first half, WVU crossed halfcourt with the ball and a chance to gain momentum going into the second half. I thought Georgetown played fine defense all the way to the very end, where Julian Vaughn closes out weakly on John Flowers, giving him a good look at the three.
We are going to join the play halfway through, after Truck Bryant has kicked the ball out to Kevin Jones. Georgetown closes out well on Jones and the obvious pass is to swing the ball to Flowers in the corner. Vaughn is ready for the closeout but he does not recognize the situation and react to the likely shot by Flowers.
Look at the closeout by Vaughn. In nearly any other situation, this is a solid closeout. Vaughn is on balance and able to cut off the drive, while having a hand up to challenge the shot. Flowers is not that great of an outside shooter, so Vaughn does not look very worried about him taking the 3.
However, in this situation, the closeout could have been much better. With the amount of time left, it is unlikely that Flowers is going to do anything but shoot. Vaughn needed to sprint at Flowers and be in his face to challenge a possible shot. Any other action by Flowers likely would have seen the halftime buzzer sound before the shot, so Flowers needed to launch the shot quickly.
With 15:00 minutes left in the first half, that is a textbook closeout on Flowers. With less than 5 seconds left in the half, the closeout needed to be stronger to challenge the likely shot harder. Instead, Flowers had the space to launch a shot near the end of the half, which gave WVU some nice momentum heading into the second half.

