I did not see a whole lot of Purdue specific tactics. I did not see the help defenders do anything different when Smith had the ball. I saw mistakes like going under the pick and roll on Smith.
On the other hand I did see the "Purdue is guarding Barnelli with 3 players". They executed this very well. Their players were very aware of what they had to do. Rarely we made them pay. Leach did briefly in the beginning of the game. The first or second posession of the second half exemplified this very well: the Purdue players were sagging and Leach (I believe) hits a wide open 3 pt shot.
I am not saying with all this we did not prep the game. Just, like you say, that the execution was atrocious. Players looked confused, lacking confidence. It's easy to see good prep and execution on the Purdue side when they are killing us and not seeing it on our when we are stinking so badly.