I've put a variety of shots on a variety of deer, and only three have ever dropped at the shot:

1) Extremely angled down (basically under my stand), so I clipped the spine.

2) First shot through the lungs, second shot was high and hit the spine - dropped on the second shot.

3) Double-lung shot. This one is the one I think about, and I think I caught him off guard - it was absolutely last light, and he was just strolling into a greenfield, certain that the hunters had gone home for the night. That was a great feeling.