DNA testing is dead, long live DNA. DNA is infallibale, right? Not when we have Hybrid Humans (NPR audio) also know as Natural Chimerism.
To make a complicated story short, some people have more than one set of DNA within them. To quote the first link:
Because these Chimeras have TWO unique types of DNA in their bodies, they can potentially, for example, have a kidney that is one type of DNA, ovaries that contain another type of DNA, and their blood may have a different type than their ovaries and so on.No one has any idea how common of occurance this is. Even you could have more than one set of DNA floating about in your body. Just like finger prints, nothing is absolute, and the reality is once again stranger than we could imagine.