It happened to me (never thought it would)
 

I shot a nice little buck (looked to be an 8 point). Knocked him down, he started kicking. I thought well I got you, next thing I know the deer stands up and runs off.. I go to the POI and there is a pile of hair, more hair than i have ever seen before. No blood. no meat or anything. My buddy and I searched for a few hours and couldnt find anything. What do you think happened?

What kind of weapon were you using? The only thing i can think of is you hit him in the Rack....rattled his head for a minute then he got up and ran when he could see straight again. And the hair could have just been from the joult! But im just guessing....

That's why you always keep your gun on an animal you've just taken a shot at until you know that you know that you know. If a muzzleloader, reload as quickly as possible.

What bullet?

this is common fer shots close to the spine...basically ya knocked him out fer awhile.
make sure your rifle is at zero fer the distance you are shooting..i'd imagine that you shot about 1/2 the distance of your zero, the bullet was on the "up", making for a bad shot.

What was the position of the deer? Many years ago I had the same thing happen. I took a frontal chest shot with my 12ga but shanked the shot left and ended up grazing the deer down the side. Must have burned like hell or something because he rolled around a bit, then got up and ran. Tons of hair, no blood or tissue. Found out I only grazed him because he ran straight to my Dad who shot and killed him.

If we're talking about a high-power rifle, we should only be seeing a difference of a couple of inches for a closer shot (assuming he is zeroed at 100).

at 25 yards..my mil is a tad bit higher than a couple inches. enough to miss a yote if ya tried with cross hairs.

Well with my 300WSM it will only shoot maybe an inch or two up or down from 20 yards to 200 yards...

i put the x hairs on a yote, and boom, went right over his back.

Describe the hair a bit more. Does it look like the hair was from all over his body from thrashing? clumps? The hair can tell you a lot more by its length and thickness. Just compare the hair to the hair you find on the body of other deer you've harvested before.

That usually applies more to clumps you find because clumps are generally from the shot, whereas hair all over is usually from thrashing and doesn't tell you as much.

Did you see the initial impact very well? A spine-shocked animal will usually stiffen right away before falling and then the kicking or struggling will begin.