(Attention: I started to play Pokémon with 6 years old, I'm portuguese and in that time i didn't even know a single word in english, so..)
My Pokémon Red mistakes:
- Name my character, the rival and my starter the same names.
- Didn't know how to catch pokémon, the first one was Digglt in Digglet cave.
- Throw away all tm's, thinkink they didn't do nothing (The first one I used was Fissure, one of the last we receive..)
- Put a Magikarp at day care, forget me of that for a long time.. it goes from the level 7 to 86.
- Didn't know how to work with boxes: I release my Eevee when I was trying to put it on my party.
- Didn't catch Zapdos and Moltres, thinking they're common pokémons.
- Do the Missingno glitch to get infinite Masterballs. When I face Mewtwo, I only have 'one' masterball (I didn't noticed it's 100!), so, I didn't catch Mewtwo.
- Hiper Beam rocks! I put that attack in all of my pokémons that could learn it.
- Do the Missigno Glitch with Old Rod on 6th place.. now I can't place it on pc storage.
- Go to safari zone and catch all pokémons that appear, because I thought if I run, I got out of safari zone. Result: I have a lot of Nidorans.
And I think that are 'only' these ones :)