My worst moment, was when in my haste to complete my pokedex, I traded half my decent pokemon to my other game (including my lvl 100 Charizard), started a new game, saved it, then later realised what I had done. I now call this event "the great delete".
I got bored of Pokémon in the summer of 2000. In the winter, I decided to have a play on my old game, for the memories. I turned on my Gameboy to find that the file data had corrupted, and I'd lost my complete Pokédex and my level 100 team.
Oh, I also once ran away from Articuno, because I didn't know that there was only one.
When I got pokemon yellow, even after completing the game, I was still totaly naive about it, and this guy convinced me I should trade my four level 100 pokemon for some lame pokemon of his from blue you couldn't get in yellow. It totaly was not worth it.
When I started my game... I didn't know how to get out of the house
In the Team Rocket hideout thing. I was easily confused by the warp panels
-_-; I was such a dumb kid back then. I thought that I was permantly stuck in that one place that I erased my file with my lv.50 pokemon to get out ;_; (I overtrained my pokemon)
My worst moment was when I was trying to beat the Elite Four. After many tries, I finally beat Lance, just barely. I thought I had won the whole game, but then suddenly I realised I had to battle Gary. Then I lost to Gary. I was so annoyed...