Right now the only thing I can think of is thinking that Lickitung was a gen 2 pokemon and arguing over my older brother about it XC
Now that I think about it more, I remember some things I thought were untrue about typing due to gen 1 pokemon games. Like, I used to think they made Psychic types way more OP in terms of what they are actually super effective against. Like due to most grass types also being poison types & the only ghost types around were poison types, I thought Psychic & Ghost were super effective against each other (-_- what would be the point in that...) & that they were also super effective against grass types. I actually argued this also with my older bro & went out of my way to fight a Tangela to prove him wrong(but I was pretty embarrassed to find out otherwise), because back then it just made since for Psychic to be super effective to grass types to me for some reason.