Bone marrow transplants can cause false positive matches with criminal DNA testing. For example, there was

...an Alaska case in which a man was linked to an attack, based on DNA obtained with blood tests, but had been in jail at the time.

It transpired the sex attack had been carried out by his brother, who had donated bone marrow to his sibling in a transplant some years earlier.

More details here. (Via SciTech Daily.)