The majority (if not all)of "Marginal" hits that result in quick deaths revolve around arteries.
You hit a major artery ANYWHERE in a deer and a quick death is a certainty.
A friend hit a doe last year square in the hindquarter after his arrow deflected off a limb.....the result? The bloodiest shortest trail he's ever had.
These arteries travel throughout the deer's entire body and some pretty weird hits can take one out. The bloodiest trail I've ever had was on a doe that I hit the artery running along the spine......the animal ducked at the shot and I caught her high. I NEVER saw blood spray like that in my life, and the deer went 20 yards.
Obviously the other "Marginal" hits that can result in quick death are in the central nervous system, especially the brain.

But of course everyone knows that these shots should not be attempted on purpose.....but they CAn happen and you asked.