1. When I played Diamond, which was my first game, I had six Pokemon in my team and I caught a Pachirisu, it said it was sent to someone's PC. Now I thought that meant that Pachirisu was sent to someone's computer in real-life and they would suddenly get that Pachirisu. Later I found the PC in the Pokemon Center after talking to some guy. I still laugh at that.
2. Thinking that rapidly pressing a A and B would increase your chance to capture a Pokemon. The reason is because in SS I caught my Mewtwo while doing that.
3. Thinking that Milotic and Feebas were Sinnoh Pokemon even though a knew some Pokemon were from other regions.
4. Using a Lv.27 Jolteon in the Pokemon League. Weirdly, that was the first time I ever won the Pokemon League after like 10 tries.
5. After reading it somewhere, I thought that bringing a certain team of 6 Psychic Pokemon to Spear Pillar, Mew would appear.
6. After reading it somewhere, I tried putting 2 Ditto in the Daycare with one holding a Master Ball, that it would make a egg that hatched into a Ditto holding a Master Ball.
7. Thinking that if you battle the Pokemon League again, you would lose all the progress you made after beating the Pokemon League the first time.
8. Thinking that Absol was a Legendary.