I saw a TV documentary a few months back about a mother who was put through hell. She had a paternity test done, and the authorities proceeded to rake her over the coals; the test determined that her husband was the father, but she could not be the mother. After a prolonged nightmare, someone involved stumbled on a similar case, and it led to further testing and the actual answer. The mother is a chimera. In medical terms, that means that different dna can be found in different tissues of her body.

I have a suspicion that this has been looked for so little that it may turn out to not be all that rare. It's another way that the paternity test results could get skewed some.