From a guides aspect of hunt killers here it is and not for the weak of heart.

No.1 Is being so out of shape you can't even make it 400yds without dying in the field. ( Have to be able to get to the game)

#2 Bringing a brand new unfamiliar rifle that cost's morethan the hunt that the person hasn't fired a box of shells through.

#3 Unsighted rifles that gunsmith boresighted just before their hunt or a buddy sighted in for them.

#4 Scopes that have more gadget modes than a gps.

#5 Forgetting the guide told you to turn down your scope to lower power while hunting in the trees.

#6 having the focus ring on the scope set at long yardage while walking in the trees.
#7 using more than one type of ammo on a hunt (opps)
#8 Talking in a normal loud voice (Grrrrrrrrrr)
#9 Playing with the gps instead of having four sets of eyes looking for game.
#10 IF NOT THE BIGGEST HUNT KILLER OUT THERE> Losing self control not cycling a round into the chamber. Shooting at hair or somewhere other than the animal. And most important so excited it makes the situation not safe.

Mostly from a guides aspect besides not being in shape the most important boils down to shooting. The rest is usually being handled by the guide. Of all the hunters I have personally guided only a very small select number of them where actually what I would call seasoned killers. The man that can step of the horse or stop in mid stride and aim, relax and kill the animal with no secound thought of all the what if's in the field of hunting.