A lot of mine were back when I played Red Version and was therefore new at Pokémon. For example...
- I started out with Charmander and had no idea how type matchups worked. Naturally, I was wondering why Brock and Misty were so tough for me.
- My first Charizard had Fire Blast, Flamethrower, Cut, and Ember. How diverse!
- When I saw Articuno in Seafoam Islands, I didn't know that you only had one chance to catch it, so I talked to it, thinking it was like any other Pokémon on the field (like those in houses). Naturally, I wasn't well prepared and missed out on it. I didn't save before it, either.
-- When I found Zapdos, I recall underestimating it and probably only had a few Ultra Balls and not anything with sleep moves, and I didn't save beforehand.
-- When I found Moltres, I had more Ultra Balls, but when I fought it, it turned out the current box in the PC was full! No, I didn't save here either. How embarrassing...
-- I did catch Mewtwo though, since I did think to save this time and was lucky by catching it with a few Ultra Balls.
- When I played the first time, I used my only Master Ball (this was before I knew about the MissingNo. glitch) on some weak Pokémon. I think it was a male Nidoran, not knowing how valuable the Master Ball was. Oops.
- The first time that I played Crystal, in the Dragon Shrine, I answered one of the questions incorrectly (IIRC, when I was asked "What kind of Trainer do you wish to battle?", I answered "Tough person" instead of "anybody"). So, I sadly missed out on the Extremespeed Dratini
Although, the second time I played, as well as when I played HeartGold, I was more careful to answer everything correctly.
Thankfully, I never believed in any of the rumors like using Strength on the truck, non-existent Pokémon like "Pikablu," or Pokégods. I was always skeptical about those because nobody could ever prove them to me. (Although, I didn't believe in MissingNo. either at first until someone did show the glitch to me.)