What's the biggest rookie move and mistake you made when you played this game for the first time? For me, it was on Pokemon Yellow, and I was battling Brock's Onix, and I only had my Pikachu with me, so I just kept ordering Pikachu to quick attack Onix until he fell. Didn't know there are more effective types, like a Mankey fighting type or a Butterfree bug type that can easily beat Brock in a few seconds. Me, I spent about 30 minutes battling and resetting the game after I lost. What about you?
Only leveling up your starting pokémon would be my biggest mistake by far. I could never help myself, they're just so cute when you start out, yaknow?
My biggest mistake was definitely using the master ball against Dialga in Pokemon Diamond. A time later, I realized that Palkia/Dialga, in their respective versions, had a higher capture rate than other legendary pokemon. Maybe it doesn't apply in other versions, but with Diamond and Pearl, it does.
Not packing enough pokeballs or potions would be my mistake. It's hard to get when you're just starting out but if you go to the jungles and safaris enough you can earn it. I have a tendency to be a bit greedy when I'm starting a game so that's a big problem for me. I also underestimate the damage of other pokemons so mine die before I get the chance to use potions.
Saving inside a cave with my only Pokémon left in critical health, without potions or escape ropes....
Same! I would only level up my charmander and I would think that I was such a smart move, but when he died, i was totally screwed. But I learned it quite fast and never made that mistake again.
I would have to say that one of the biggest rookie mistakes I made was selling rare candies at the Pokemart.