Entertain this scenario, if you will ...
I've become bored with dressage, and decide I want to take up eventing. In order to do that, my mare Lyric will need to learn to jump. (Keep in mind that this is totally hypothetical, for three reasons: 1) My mare jumps like a goat. 2) I haven't cleared a fence on a horse in the better part of two decades. 3) In that time, I've managed to forget quite a lot of what little I knew about jumping.) So - keeping reason #3 in mind - I take her to a trainer so she can learn to negotiate fences. Said trainer mounts, starts trotting her over some ground poles, then progresses to cavalettis. All's going well, and the trainer is about to start with some cross rails. Suddenly, what little knowledge of work over fences I have retained percolates to the surface, and I morph into what I call a Self-Appointed Expert. I butt in with, "Don't you think those cross rails are too high/low/wide/narrow? Shouldn't you put a ground pole down? I don't like the distance between the cross rails - it's too long/short. And the poles are a funny color. Why don't you put some wings on that cross rail? Lyric won't go straight into it without wings on it. And why are you using those white plastic blocks instead of real jump standards? And here, let me lengthen your stirrups - they're far too short for riding cross rails. And watch your hands - I don't want you hitting my mare in the mouth. Why are you trotting/cantering instead of cantering/trotting? And I think you should have a line of 6 cross rails rather than 3 - or better yet, four cross rails and two verticals. And wait - I don't like the footing here. Can't we drag the arena/move to a different arena? And why are other people working in the arena? Lyric can't concentrate with other horses in here."
Now, in my mind, such behavior would justify the trainer dismounting, handing my mare off to her assistant, and pounding me to death with a jump cup. In real life, though, the trainer might answer all my concerns calmly ... and then discover that her schedule was far too jammed to accommodate working with my horse. She'd be very sorry, she'd recommend that I try Trainer X just down the road ... and as soon as my tail lights were out of sight, she'd punch a hole in the tack room wall and call Trainer X to warn her about me.
My scenario is quite over the top, I know; a lampoon of a little knowledge being a dangerous thing. I do have customers who involve their trainer, their farrier, their vet, their equine massage therapist / chiropractor / acupuncturist / communicator in the saddle fitting process - and to be perfectly honest, I know people in most of those fields whose input and opinion I accept without question. There are times when input from a professional who knows the horse and/or rider can be incredibly helpful. If I note that the horse seems a little off, the vet or chiro can tell me that the horse has arthritic changes in the left fore fetlock or issues with the sacroilliac, and I can stop worrying that the saddle is causing the issue. Trainers can tell me if the horse's reluctance to move out energetically (or prediliction to move out too energetically) is due to past training or pilot error, or if it might be the saddle; they can tell me if the rider is leaning behind the vertical out of habit or because the twist is wrong, and they can tell me if the head toss going into the canter is habit or something completely new.
Self-Appointed Experts (hereinafter known as S-AEs), on the other hand ... Their area of expertise suddenly widens to encompass saddle fitting (and perhaps even saddle design and manufacture as well), and they make a saddle fitter's life absolute hell. I may have found a saddle that perfectly suits both the horse and rider, but if it doesn't make the S-AE happy (and S-AEs are, by definition, almost impossible to please), it will not be accepted. And unfortunately, many customers who have Self-Appointed experts in tow are firmly under the Self-Appointed Expert's influence, so my opinion on the whole process is just so much barking at the moon.
With S-AEs , there's no sense in arguing. Gods know I've tried, but fortunately, I was wise enough to not spend too long beating my head against that particular brick wall. I wound up looking like a contentious no-nothing who was just jealous of the wealth of knowledge available to the S-AE. In those cases, the customer leaves with whichever saddle the S-AE chooses and I wish them a safe trip home. End of story.
Or, more usually, not.
Almost invariably, the customer will come to his/her senses at some point ... and often, they come back. If we're lucky, it didn't take long, so the horse will only have been made mildly uncomfortable. But if we're not, there can be serious training or behavior issues (such as bucking, bolting from the mounting block, biting or kicking when approached with the saddle, or refusing jumps) or serious physical issues such as muscle atrophy or alignment problems. Serious problems always make fitting tougher. For one thing, you need to wait until the horse is no longer in pain - you can't do a saddle fitting on an unsound or injured horse. And when the horse is finally healthy enough to do the fitting, you run into tons of questions. Is the horse trying to nip while being girthed because he's uncomfortable, or is it habit and remembered pain? Is the horse not picking up the left lead canter because of saddle fitting issues, or because muscle atrophy makes the left lead incredibly difficult? Is the horse's gait short and choppy because of the way this saddle fits, or is it because of the way the last saddle fit?
Karma will at some point catch up to the S-AE. Best we can hope is that it's in the form of another S-AE who runs roughshod over their particular area of knowledge. Not that the first will necessarily learn anything from the experience, or gain sudden insight into their behavior ... but it's satisfying to think they might get a taste of their own medicine.