I'm curious what could happen if someone accidentally had a child with an unknown cousin?

Risks: Geneticists estimate that we all carry silent recessive gene defects.If paired with another carrier it could produce chronic disease (like cystic fibrosis) or death( metabolic disorder).Marriage between close relatives increases this risk. Marrying outside the family decreases risk for rare afflictions.Your risk could be as high as 25% for if you both carry the same recessive mutations.