The fact that some people can be born with 6 fingers doesn't mean that saying "a human hand has 5 fingers" is generally false.
It also means you can't use the generalization "a human hand has 5 fingers" to then say that "therefore because your hands have 6 fingers, you are not a human"
It also means you can't use the generalization "a human hand has 5 fingers" to then say that "therefore because your hands have 6 fingers, you are not a human"