I'm really not sure how you'd get the sex of a cat wrong. They don't exactly try to hide it. :|
It can still be pretty hard. We once had some kittens that we named before finding out we got their genders wrong, meaning one of the girl cats ended up being named Jeff.
Kittens are a bit different story from essentially full-grown cats as is case here though. There's this slight matter of rather obvious external primary sexual characteristics in the males (and conversely due lack of family jewels in the females), as with pretty much all mammalian quadrupeds...