Everyone calls Mufasa a girl - when you can clearly see his "thing."
Kiara is often called a boy, although she always has pink or purple collars.
People mistake Nala for a boy, I suppose because of her size?
Simba is never mistaken - he's 100% handsome male!

It doesn't bother me that much, I just correct them politely, and they usually never make the mistake again.