Khaith (my kitty^^) is always mistaken for a female.. and they always think his name is Kate. Well, it doesn't help much when I used to let him wear pink collars.
Frisk... everyone tends to mistake him for a girl. Anyone who knows my fat little boy says that they couldn't picture him as a girl. Maybe his name is a bit feminine? Or maybe he looks a wee pregnant(He's not round enough in my opinion to even come close to looking pregnant, though)?
People get the genders right on everyone else, though.