Since red/blue came out I have never truly associated flying types with normal. I just can't ever get my mind around it and it drives me crazy. I always see the birds as flying, flying/water, flying/bug, flying/whatever..... NEVER FLYING/NORMAL!!!
"That which does not become part of the one, shall become one with the void."
Darkrai is purely Dark, not Ghost, or Dark/Ghost. Groudon is not Ground/Fire. Azurill is Normal, not Water. Gastrodon is Water/Ground (this got me a lot in Pokémon Mystery Dungeon.) Mawhile is not Dark, but Steel. Darmanitan is pure Fire, not Fire/Fighting.
These are the only ones that have ever really bothered me.
Beheeyem isn't part Rock or Ground. Drapion isn't part bug. I didn't realize Magnetmite was part Steel until Gen 3 (though this isn't entirely my fault). Didn't know Lugia was Water/Psychic until like, Gen 4. Took me a bit to realize Groudon was part Ground.
I also swore Psyduck/Golduck were part psychic until this gen, since the pokedex is always talking about how it has psychic powers. I constantly think that Lugia is Dragon/Water, and that Darkrai is Ghost/Dark. Drapion is definitely the worst though, I even USED one in the 4th gen, and still couldn't get it down until this one. Finally was able to remember it by telling myself that it ISN'T the type it obviously looks like it should be to me, which is Bug/something.