When I was learning about all the new Pokemon from Generation 4 on the internet, (I didn't own any Pokemon games yet and I don't even know if Diamond and Pearl were out yet) one website said that Shellos was supposed to evolve into Lapras. It said something like "Used to be Lapras's baby form". All this ever did was confuse me. Of course, no official sources had ever said it was supposed evolve into Lapras, but I didn't know that back then. I was too young to know any better.
I watched the anime before I played the games, so the first time I played Pokemon Red I thought my Charmander could win against Brock's Rock Pokemon through sheer willpower or exploiting a weakness. After limping my way out of the Pokemon Center for the tenth time, I broke down and caught a Nidoran to Double Kick them to death.
Actually, the very FIRST game I played was Pokemon Silver (bought a Gameboy Color for it and everything!), but I couldn't figure out how to switch the first Pokemon in my roster out with a different Pokemon, so I was frustrated that my starter would always come out first and mop up all or most of the experience points, leaving it over-leveled and my other Pokemon under-leveled. I played through the whole game with each starter before I thought to ask one of my classmates how to switch the first slot with a different Pokemon. I was so embarrassed when they showed me the easy way I'd overlooked.
When Gen 3 games came out, I thought Torchic would learn a steel claw move the way Charmander does in the FR/LG remakes, so I kept leveling up waiting for it to learn a Steel move to use against Roxanne. Before it evolved (where I would have discovered Combusken learns Double Kick), I broke down and caught a Lotad, then it got its under-leveled tail kicked time and time again until it evolved into a Lombre, when I FINALLY won my first Gym Badge at the 36 hour mark. I was so embarrassed by this mistake that for years after I would speed run through all my games, not wanting to take forever like I did on that first Pokemon Sapphire playthrough.
Anime-wise, before Johto Pokemon were officially revealed, I believed the rumor that the round mouse Pokemon (Merrill) was called "Pika-Blue," a new, alternate evolution to Pikachu.
Didn't know what shiny pokemon were so started a new game when my charmander was this weird color and this weird sound played when it came out thinking it was a glitch. It was the first pokemon to appear in a brand new leaf green game. It wasn't until later where my brother kept running into these weird gold poochyenas did we learn what a shiny pokemon was.
I guess I didn't always think about type advantages and disadvantages. As a result I got a lot of them mixed up, and it made for some shocking battle experiences. (Yellow and Crystal)
Believed Missingno was a real Pokemon.
Does anybody remember the old rumor about the town you could supposedly go to after you beat red and blue where people traded pokegods someone even provided a fan-made Sprite of the town which is basically a clone of Celadon City but instead of some of the buildings were small all the buildings or at least 3 or 7 werected alternate versions of the department store