Let's see. I didn't realize that the first two Nidos are pure poison type, and that only Niodking and Nidoqueen are Ground/Poison. That would explain why Surf wasn't SE.
I'd been playing a lot of Yellow recently, so when I switched to Crystal, I forgot that my Croconaw could beat Morty up easily with Bite. (I still thought that Bite was Normal-type.)
i sorely wished that Magnemite was part Steel in Yellow as I had Double Kick Niodrino, but alas, it was only Electric.
I was pissed to find out that Gust was Normal type in Gen I, so I ditched Pidgey because it would only learn a decent Flying type attack at Level 28 (if i allowed it to evolve, Pidegot wouldn't learn it until Level 33). Meanwhile, the Level 10 Spearow I just caught already had Peck on it.
Drapion tends to fake me out, as do the two fossil Pokemon from Gen3. I can never remember their second typing, even though it's not exactly subtle (grass for the sea lily, bug for the proto-arthropod). For the longest time, I also thought Suicune was Ice and not Water, for some strange reason: probably accidental conflation of the Beast Trio with the Bird Trio.
All above is completely true and accurate, unless it isn't.
I have never had a hard time keeping Pokemon's types straight-- until Gen 5. I keep thiking the Gothitelle line is a group of DARK types, not psychic. I thought it was a counterpart to the Solosis line (since one is found in White and the other in Black). I thought they had a Dark type in one and a Psychic type in the other. But no. I have been consistently wrong. D: