I have only had 2 drop on the spot, both whitetail bucks, both shot with my .270. The first, shot at about 25 yds, hit in the heart and lungs, dropped in his tracks, and then slid and rolled about 40' down the side of a ravine. Talk about fun trying to drag him out of there, I should have let him take a couple steps closer away from the edge. The second buck was running at about 150 yds and I hit him too far back and high, but took out a hip and his spine and evidently some major blood vessels, he flopped over in midstride and stayed down. When I got to him he was still alive (could move his head and forelegs a little) so I popped him at the base of the skull with my pistol. I think if it had taken me another 30 seconds to get there he would have bled out and been dead anyway. As a contrast to this my buck this year ran full spead for at least 60 yds with his heart literally blown in half before he piled up.
Most that I have seen drop on the spot involved either central nervous system damage or were firmly anchored by a quartering shot passing through vital organs and taking out the far shoulder with all of the bullet's energy being transfered. I also believe that a calm, standing deer is much more likely to drop than one that is already pumped full of adreneline.