For me, personally, the worst I did was that I applied the wrong training method (correction type) to my dog. He's sensitive, slightly skittish, and thinks too much, and using corrections, like yelling, snapping the leash, using intimidation, etc., was too much for him. Even little things would be too much for him. So it's no wonder he was a nut-head when he was, I think, 6 years old and was starting to nip at everyone and lose his temper of the drop of a hat. So I switched to positive-only and there was an immediately turnaround. Now, he nips very rarely (because of fear-based reactions, which was the deal before, too, but now he just gets nippy when you're leaning over him, tense, while readying yourself to clip his nails, which he already hates. He's sensitive, so he picks up on your body language SO EASILY!). He's now 10 years old and grumpiness is making an appearance again, all due to his age. No nipping at people, but sometimes his "trigger" goes off sooner than usual and he'll lunge at one of the cats, trying to get her to move so he can do something rather than just stare at her. He's also warty and bumpy and I don't think he likes it when I'm playing around with them. But yeah, we used fear-based training and intimidation in his early years and it messed him up. I remember our puppy obedience instructor advocated the scruff-shake, but in a more intense way. See, when the dog is getting into a bad mood or bad behaviour (and Jake was the bad kid of the class, always trying to walk around and meet the other pups, getting into trouble, growling and tugging the leash all through the lesson, etc. Not evil, just distracted and a normal puppy). So Jake was the guinea pig when the instructor demonstrated the reprimand. What she'd do is grab his scruff on both sides, lift him up so his eyes were level with yours, then shake him and yell at him ("No! Bad dog! I'm the boss!"). Jake was TERRIFIED and would snarl some more, showing his teeth, all with his little tail tucked tight between his legs. Is it any wonder that it took years before I got it into my head that him showing his lips if you even touched his collar wasn't normal (it was quickly solved and now he doesn't have a problem being touched around the neck). He's also afraid of heights and will completely freak out if you try to pick him up (including using his teeth; he got mom in the lip in that last year before I made a change to go positive-only, and early in his life, I remember he somehow gave me a goose-egg in the BACK of my head. I'm still not sure how he managed that). He's also supersensitive to yelling, and even if the yelling isn't aimed at him, he'll slink and find a small, dark place to curl up. Unless he sees the yeller looking at him, and he'll assume the yelling is about him and he'll slink and crawl up to the person, ears down, tail tucked, calming signals screaming out of his body. Yeah, that one lady and that one training method messed Jake up good, but I've worked on him and I'm satisfied with where he's at now. I can deal with not picking him up (55lbs isn't fun to pick up anyway, lol), so I work around it by training him to reach higher ground himself (he was fearful of the bathtub for a while, but now it's easy to get him to jump in there by himself rather than having someone heave him up and put him in there)
I'm not sure exactly where this lies, but I wish people who get dogs would actually make an effort to get to understand him. Figure out his likes and dislikes, and how he operates, and then apply it to his training. It makes things so much easier for you and the dog, rather than picking some random and easy method to training a certain trick. Part of the reason I couldn't get Jake to learn rollover until he was 6 (and he's a Border Collie-Australian Shepherd mix, so the brain is there) was because I was going about it the wrong way. I tried everything, from manually pushing him over (he hates that kind of thing, he'll even freeze and not move when you command him to sit and you've got your hand on his shoulder), and guiding his head with a treat and hoping gravity would roll him over. See, Jake's the kind of dog that learns through experience. He has to figure it out himself, and if you push him over, he doesn't associate that with the word "roll over." To him, you just pushed him over and then said "good boy." It confuses this old man. I did eventually teach him roll over, and it was all becaue of a fluke. We were out in the front yard with bits of cheese slices (he loves cheese) practicing random tricks. He was in a liedown at one point, and I told him to "roll over". I can't remember if he was at the point where he was getting frustrated (not understanding what rollover meant, so trying different tricks until he picked the right one), and eventually went into play-dead, or if he just did it. But anyway, he was in a lie down and went into the play-dead mode (laying sort of on his back with his forelegs lifted in the air). As it was, the whole fluke thing, he was laying on a sort of hill, so when he lifted his legs (and probably arched his back), gravity rolled him over. I started leaping up and down, squealing, and all but forcing cheese down his throat, and I saw the lightbulb go off in his head. He was grinning, and I could just see his head working. I brought him back to the exact same spot, commanded a rollover again, and he immediately went right into a roll over. I kept repeating it, giving him praise, a few times before I tried a different spot that was level, not a hill, and he rolled over perfectly each time. He's been flawless ever since. Nowadays, he'll only roll over on random days, due to his age. He has arthritic hips (which don't even affect him in day-to-day life, he doesn't even need glucosamine anymore), and he needs soft ground, and just general comfort. Otherwise, when he tries and feels hard ground, he'll get back up and won't attempt it. He's an old guy, so I just go with the flow and do something else. He's my first dog, so I've done a lot of experiments on him (including the corrections method of training for years), so he deserves some flexibility at this point.
Yeah, the training of sensitive dogs is one of my bigger issues, if you can't tell, lol. Most of those training methods out there just don't work on the more sensitive dogs and can just make things worse. But nearly any positive-only training method works with them, and that method is becoming more and more popular these days.