Mistakes were made

I don't know if I would consider these actions wrong, just some things I did that I now regret a little.
1. Evolving high IV, low CP mons and leveling them up
2. Actually leveling up a Kingler, a Starmie, a Magneton, a Voltorb back when I first started playing
3. Leveled up 2 Gyarados with 98IV with Bite/Dragon Pulse to Level 30
4. Leveling up 3-4 Dragonites with sub-optimal movesets to Level 30. SW/DC, DB/HB
5. Leveled up 6 Snorlaxes with 100-91IVs to Level 30 to use as both attackers and defenders
6. Wasted my guaranteed Eevee evolutions (gen 1 ones) on low Level, high CP Eevees

So many mistakes, oh and I'm still making them.
Twice this past week, I've meant to hit the "x" but hit the evolve, then not realizing what I did, hit "yes". So painful watching that 30 seconds of evolve while saying "NOOOOOOOOO!"

Tranferring mons because of a "bad" moveset, when after one of many rebalances that moveset is now the best.
not expanding my mon bag sooner (see above)
not expanding my items bag sooner.
Powering up mons that had no business being powered up.
Evolving some mons just for the XP, then transferring. Should have saved that candy for potential future uses. (I no longer evolve just for XP any more)
Not traveling to a biome that had the possibility of finding chanseys during the Pink event. Seriously lacking Chansey candy.

It's difficult to answer this question. Technically, any evolution before level 20, or power-up before 30 was a mistake, but I would have lost interest if I hadn't done those things. I candied many DG wigglies, before gen2. It took me a while to get on-board with movesets and IV's, which was also sub-optimal. I also wish I had bought the go plus sooner. More recently, I've missed a couple 7-day streaks.

At the end of the day, I'm happy with where I am in the game and overall I've enjoyed my experience, so I don't really have any regrets.

1- Powering up Pidgey back when started.

2- Poweing up Flareon.

3- Evolving starters and some others just for the mass evolve lucky egg time.

4- Powering up Raichu.

5- Powering up a low IV and bad move set Lapras.

Not sure if any more right now.

so Far, my worst mistake... transfer my first starter pokemon... ya, my charmander... i never feel any mistake when im powerup my pinsir or other pokemon low or high tier... my pokemon my rules...

God knows what I transferred with low CP but great IVs early on. I know I transferred 10 to maybe 20 BS Snorlaxes with low CP and unknown and sometimes good IVs before the moveset changes.

I slightly regret spending too much dust on Venusaur and Starmie. They are decent attackers but there was no need for me to push them beyond level 30.

I occasionally powered up crap like Persian, which didn't even have Scratch. And an Ivysaur, it has VW/SoB though and I like it more as it is, wouldn't want to look at a Venusaur's wide backside while attacking/prestiging. Walked with Pikachu back when the tracking system was crap and I couldn't find more to evolve, the Raichu has good moves tho. Powered up a Wigglytuff and Clefable below 60% because they had good movesets, now the Clefable is legacy so I guess that's all right. I'm a bit of a perfectionist but can be satisfied with 70%+ 'mons if they have good moves. I spend dust as soon as I get enough to power up a 'mon I currently favor, but I guess that's just my playstyle. And yes, for me a Bellossom with all grass moves is worth powering up 'cos I expected Acid as quick move and was going to just walk it to get fast candy for another try. It's simply showing gratitude to the gods of random, imo. You get a 'mon that makes you happy, you invest in it. Pokemon is about cherishing your monsters and making best use of what you have, not about bragging. I hated that "breeding for shininess" and similar crap people do in the original series, you need to have a clockwork heart and nerves of steel to do that, and if you're that much of a machine, why even play games and not invest your time in reality?

Transfer a 100% IV Magnemite with low CP, probably.

Probably the worst was powering up my only hatched chansey (they do not spawn here EVER!) to level 25.5, using all my candy. At the time, I was clueless about blissey, what can I say....

P.S. Chansey and I will be walking for a long, long time - she found only 18 candy ;)