This is a very useful exercise! I try to be pretty forgiving of mistakes, because usually I can learn something from them. I'd rather err on the side of experimentation than stagnation, even if that means making mistakes along the way. I've tended to think of my mistakes as one-offs, but there are a few notable patterns:

1. Thinking I had to pick a side between black and brown, as well as black and navy.
This old-fashioned style rule tripped me up for a long time, and resulted in a sort of pendulum swing between black and other dark neutrals. When all my black was gone, I missed it! But if I added it back in without maintaining a variety of other dark neutrals on the side, I missed those other colours! Dark neutrals are happy colours for me, and I get bored easily without variety, so it's better to have a smattering of all of them: black, brown, navy and charcoal, along with olive, pine, eggplant, plum and burgundy, are all represented in my current wardrobe. My dark neutral rainbow makes it easier for me to get dressed, not harder.

2. Purging things when I get bored with them. I don't do this nearly as much as I used to, mostly because I have a larger space for my holding zone. I'm usually pretty good about wearing things into the ground, but I can think of a handful of things I let go of too soon, simply because I didn't feel like wearing them at the time and thought I ought to "make room" for something else. Examples: a sage green cable-knit wool hoodie (much higher quality than most of the knitwear out there now); an asphalt grey tencel-blend midi skirt (thought flowy skirts weren't for me, but this one was so comfortable, and the colour made it feel more minimalist, less boho); a vintage turquoise buffalo-check wool midi skirt (part of the same skirt purge; decided it wasn't versatile enough, but looking back I shouldn't have let it go because it was so unique); a perfectly tailored white/tan pinstriped oxford shirt (thought I didn't like tan and browns -- hah!).

3. Trying to find "my" version of things I don't really like. The first thing that comes to mind are the leopard print items below. I'm not sure why I get it in my head every now and then that I ought to like things I don't. The challenge of solving a puzzle? A craving for variety? I got a lot of compliments on those leopard print items when I wore them, but even though they looked fine neither piece felt authentic. Other examples are mostly footwear: ballet flats, loafers, Birkenstocks, Stan Smiths, riding boots... I've spent a lot of time trying on shoes that I don't really like even though they looked great on someone else. Usually I manage not to buy them, but not always!