With .6 seconds on the clock, the Celtics lined up for an opportunity to tie... or win the game with a 3. With Paul Pierce taking the ball out, the Celtics set up for what appeared to be a double screen for Ray Allen. Brian Scalabrine was on the floor as a safety valve and is staying far enough away to leave space in the middle.
Instead of setting screens for Ray, he clears out to the 3 point line. Since he's such an obvious threat, his man guards him closely. That leaves the middle wide open for what we all know happens next. Glen Davis goes out and sets a screen on Rajon's man. He sets it at an angle that allows Rondo to curl right around both his and Davis' guy... giving him a clear path to the hoop... where Paul Pierce hit him with the alley oop.
If you look at the play unfold, you can see two people on Miami are at fault for this. First and foremost, as was mentioned on the broadcast, Quentin Richardson hardly could have been any lazier guarding the out of bounds play. His hands were to his sides... he wasn't trying to distract Pierce at all... or make a pass difficult. You can see it at the :32 mark of the video.
Also at the :32 mark, look at Glen Davis picking Chalmers. Jermaine O'Neal is in the lane. As the play continues, you hear the Heat color guy say "no help" just as O'Neal rushes to Glen Davis and Chalmers is trailing the play.
O'Neal should have stayed in the lane. With .6 seconds on the clock, you don't fight through picks. You switch on ALL picks. The other team doesn't have time to make a move, so it doesn't matter if you have a mismatch. You just need someone to maybe get into a passing lane or, at the very least, bother the shot a LITTLE.
If O'Neal had called for the switch, he would have been there to challenge the alley oop and it would have never happened. Pierce MIGHT have found Davis for a jumper... or maybe Ray could have flicked up a quick 3.
But those are very low percentage shots. As great as that alley-oop play was... it was defended pretty poorly by the Heat. Maybe they were too preoccupied with the prospect of a game winning jumper... but they still could have defended a potential jumper while not allowing a clean look at an alley oop.