Oh, where to begin? Well, in my first Pokémon game, which was Firered, I found out you could catch Pikachu in Viridian Forest (the sad part is because I was ~7 years old). I was trying to find a Female one, and I found one. I didn't know what a shiny Pokémon was, but I thought all female Pikachu made a little sparkle-thing, so I caught it and trained it to a Lv 55-ish Raichu by the E4. I won with just Raichu (my second highest Pokémon was a Venesaur, and it was in its 30s or 40s). Went on to the Sevii Islands, and by the E4 rematch, Raichu was about Lv 80-90-ish. I lost against Lance. I then discovered the trade on Cinnabar Island where a guy trades you his Electrode for your Raichu. Bye-bye, shiny Raichu. I haven't found a shiny on any of my many playthroughs since.
I read something online about finding Eevee in tall grass in Firered, so I ran around for hours trying to find one. Did something similar with Swablu, who was adorable to me at the time.
I wasted my Masterball on a Lv 50-something Fearow, probably thinking Prof Oak would just give me another one.
I freaked out when my Pokémon fainted and my screen went black. I thought I had lost the game; that was it-- no more Pokémon...
I also freaked out when one of my Pokémon fainted, because I thought it was dead.
A little weird, but I ran around like crazy trying to find a bathroom, and I tried to potty-train my Pokémon, because I was really weird... Let's just say I thought the ledges were rows of.... waste.
I didn't know how to use the dowsing machine in Firered for about 2 years. When it said you should dig around with your hands, I thought the character would bend down and actually start digging.
That's all I got for Firered blunders for now; will probably be back