My family adopted a couple of cats from a shelter two months back, after one of their original cats died. One of the new cats was this glossy black kitten with a splash of white on its chest - the shelter told the 'rents that it was male, and they didn't check any further.
When I saw it for the first time, I just felt this immediate sense of... not-male-cat-ness. It kept popping into my head that the cat was a girl, or that it seemed like a girl. For absolutely no reason I could describe. And when it had its first trip to the vet... yep, it was really a girl.