what has been your biggest mistake?

Using the Eevee-loution name easter egg FAR too early. I now own a 698CP Vaporeon with terrible IVs, 435CP Jolteon and 738CP Flareon because I was so excited to get them ??

Spent some stardust before level 30 and thus wasted some as I got better Pokémon later. That's probably it.

I spent some stardust leveling up Hypno when I was like under lvl 15, however it made the game more fun for me, so I don't consider that as a big mistake. Then I leveled Nidoking just to figure out that I don't really like it. And did not use most of the lucky eggs I got for mass evolve, which is probably the biggest mistake I have made this far.

Biggest mistake is to hope that RNG isn't that bad as people say.

Walked 90km with Grimer just to get worst possible moveset on Muk. Never again.

Spen stardust and candies on the wrong exeggutor (>50%IV) because it was near the good one. From 1700 to 2000 cp

Transfering a Bite/Dragon Breath Gyrados when I meant to power him up.

(Though I'm not that torn up, as I have a Bite/Hydro and Dragon/Dragon one + 4 Twisters for Gym Bait)

Once I had a Lapras nearby and did not search for it because it was pouring rain. I regret it deeply because I still don't have a Lapras and I'm Level 29.

I guess powering up poor moveset Vileplume, Nidoqueen, debatably Poliwrath (bubble/submission) and good moveset but 80% IV Nidoking.

They were all high IV and I had nothing to fight with. Didn't care enough about moveset at the time, and double-powering Nidos was a mistake (never need both).

I don't mind though. I only had 6 fighters for a long time (a single Vaporeon and Exeggutor as well) and needed them all to get several gyms every day. Those pokecoins bought me several incubators and the 8 lucky egg package, so I think I've more than earned back on it - although I probably could have powered something with lower IV and a better moveset.

My biggest mistake has to be powering up a Nidoking with suboptimal IV and Furry Cutter from level 20 to level 30

I should have saved the eevee easter egg for later and in powered up a rattata, which just makes no sense.

And i sent away a lot of the weaker Eevees and now I have a lot of candies, but no eevee to develop xD

I powered up a 100% IV Raticate with perfect moveset. I was so excited when I hatched and evolved it that I didn't realised that it stills a Raticate.

I used 20k dust on chansey :D

I powered up a confusion/ice beam Slowbro and feint attack/play rough Wigglytuff to use as defenders, but don't ever put them in gyms because my three 2400 CP Vaporeons are much better at holding onto higher level gym slots.

Powering up a Nidoking to 1800+ and realize now they aren't good for attacking gyms nor holding gyms. It has great IVs and an ok moveset (PJ - SW) but it is basically useless in the current gym meta. Poison isn't that great right now and Nidokings get eaten up in gyms. I probably blew about 45k dust on it.

I've also powered up a (90%) Golduck to 1900+ and in the long run it is probably a big waste for defending gyms but I like him and he's very rare around here so not too many regrets there. When I think I have a bit extra dust I usually power him up once to keep him max'd out at my level. I'm sure I could have spent the dust better but he's one of my favorites and that is why you play the game right?

I have a couple.

1. I went to power up my 89 IV Poliwrath with Bubble/Submission and the app switched Poliwraths after the first power up (bug I think) and I did not catch it. Ended up powering up my 56 IV with Bubble/Ice Punch to over 2k CP (still good, but not at all what I wanted to do). And later doing it again with my Snorlax with Lick/Earthquake.
2. After waiting for months to finally catch/hatch a high IV Bulbasaur, I caught a 91 IV one and later that day, when cleaning up my inventory, I accidentally transferred it (instead of the Oddish I am certain I clicked on). 4 weeks later and I am still looking for another good IV one.