OMG THIS IS THE ULTIMATE STUPID KID BELIEF. I have no idea why I thought this, but when I saw all the shiny pokemon sprites next to their original sprites in Serebii's pokedex, I thought that that was how all pokemon looked in Sapphire and that Ruby had all the original colorations. That's why I bought Ruby despite liking Kyogre more than Groudon, not realizing that those sprites were shinies and only had a small chance to appear in either game. I wanted all the originally-colored pokemon that I had grown up with. xD What can I say, it was my first experience with any series that had two similar games released at the same time and I didn't understand what might've made them so different from each other. Had to be something dramatic like that.
I, like most people it seems, used to mash A when catching a pokemon. it always seemed to help, and whenever I didn't buttonmash, the pokemon would always break out so I was convinced it worked. after reading that it doesn't do anything, I stopped that habit and I don't miss it. xD
I thought, when catching a pokemon, that they'd get "tired" of trying to break out after a while and your chances of catching it would actually get better the more balls you threw. didn't know it was a set chance that just has to work eventually.
I thought Psychic was still supereffective against Ghost in 3rd gen because I saw some psychic-type move supereffective against my Haunter and I didn't know it was part poison. I noticed a lot of people thought psychic was supereffective against grass because of Venusaur and Vileplume and such, but at least I knew that was wrong lol.
Used to pronounce Suicune as "SOO-ih-sine." Some other post here mentioned the exact same thing. xD
In Colosseum, I always used an Altaria and it knew the move Fly. I relied on that move a LOT and it was the attack I used the most out of all my pokemon. Then when it came time to learn Sky Attack, I tried to have it forget Fly, but Colosseum's message was something like "that move is too well-learned to have it forget the move right now" and I thought that meant it was because I had used it too many times and it was drilled into my Altaria's brain. HMs aren't even used in that game so why can't we forget them??
This one may be a little confusing but I thought Rare Candies gave pokemon the experience they needed to level up and it would overflow to the next level. As a result, I gave my Absol a rare candy when it was reeeaaally close to leveling up, thinking that it would be put at the next level, still being close to leveling up. Nope, all that experience AND the rare candy were wasted lol.