Alan, Errors in DNA testing would have immense impact. They would cast doubt on the entire concept. Those percentages are very small, close to negligible. Remember that you only need one ancestral discretion, or an unknown marriage, perhaps in the early 1800s, to account or it. Keep researching your tree. 1-2% of births are suggested to be connected to an undocumented father. Remember how quickly you get back to 128 direct line ancestors.
Zaph