A DNA test for genealogical purposes can result in one or more surprises. Sometimes those surprises impact more than the tester–they can impact other living family members as well. Not all surprises are about relatives who have been dead for a hundred years. You could discover that a parent or grandparent had a relationship (and a resulting child) with someone you do not know.
Non-genealogists in your family may be impacted by this discovery.
A “surprise” cousin could result in a similar fashion if an aunt or uncle had a child that no one knows about. Other living family members may not even know about that child. It is very possible the father may not even know the child exists.
Many people find no modern “surprises” in their DNA results. Some do. Those surprises may not even come immediately. That unknown cousin may not perform a genealogical DNA test until years after you have done yours.