As you squeeze the trigger all sorts of things go wrong. And with time and number of animals shot you realise how little you actually know.
In no particular order
1) Deer haven’t read the best practice guides. I have had several deer where nowt wrong with shot placement, bullet choice, bullet performance etc. the deer didn’t know what to do. I can think of a Roe buck shot at less than 50 that I though was a complete miss. It bounded off down the tramlines as if nothing had happened. I did follow up. No blood for 50 / 60 yards so called it a miss. Since the tramlines were going in direction I wanted to go carried on walking. Another 50 yards there was the buck stone dead. Bullet (140gn Softpoint 7mm) had smashed the heart to bits. Good exit hole as well. But plugged with bits of heart, hence no blood.
Can think of several other red deer that I have shot again after no reaction to first shot. Again they were dead on first shot, but just didn’t know it.
2) mis reading wind. It doesn’t blow from left to right or right to left. Wind swirls when its close to the ground. Trees, rocks, buildings and lee side of a hill all cause turbulence. Most windrifts are shown at 10mph. Thats not a lot of wind - a gentle rustling of leaves. 20 mph gently buffers your body. Wind can easily push a bullet a few inches off target irrespective of BC and cartridge
3) angle of presentation- the kill zone is not the 2D side on view you do your DSC1 shooting test on.
Instead imagine a grapefruit suspended between the front legs of the deer or antelope. This is bundle of top of heart, aorta and other blood vessels and nerves. A bullet through this and no vet will stabd a chance. This is your target and is what you shoot at. If you get this in your minds eye, you will then know how to shoot from any angle.
4) a second is a long time. A bullet covers 100 yards in little over 1/10 of a second. That is more than enough time for a deer to flinch / move. Bow hunters under stand this, and plenty of video showing that the instant reaction is for a deer to drop and arrow pass over. I am sure that I have seen deer flinch as I squeeze the trigger. Deer respond to movement- a muzzle flash, or even the bounce of a muzzle is probably enough to start the flight reaction. I have had a number of deer where shot has gone high or further back than I aimed for.
4) misjudging range - very easy to do even with range finders, especially if its s bit windy, rain, snow, fog etc. Not so much causing a miss, but follow up and foreshortening of the range, especially if shooting across a gulley or canyon. You start looking in place that is not where the animal was. You are looking around the thistle, but actually the thistle you should be looking around is 50 yards away.
5) shot deer are really quite small, don’t run in straight lines and gravity often takes over. It is easy to loose a red deer in a gutter under a heather bush only to find it several days later when the gutter is overflowing and there is a stink and lots of flies.
6) Gremlins and human ****uperry - it happens. For me, its thinking the deer is already on the table, and forgetting my technique and the bullet misses.
With experience you build all of the above into your decision making. As a guide, you should be expecting any of the above to happen and to factor this in.
Worst is a guide who pours scorn and derision on his client for making mistakes. If you are not confident of your clients skills, you should help him (females don’t make mistakes) improve and / or get the client into a position for an easy shot. If your client is holding you back from your cull targets or your other business, don’t take out a client or a guest.