In the case of Armstrong v. Manzo, 1964, the U.S Supreme Court ruled that a natural father's rights were violated when he was not given notice or an opportunity to be heard before his parental rights were terminated by a Texas court. The child's mother and her new husband had filed for adoption without notifying Mr. Armstrong, who only learned about it after the fact. The Supreme Court held that this procedure did not meet due process requirements under the Fourteenth Amendment of the Constitution because it deprived him of his legal right as a parent without proper notification and hearing. This ruling emphasized that all parties with significant interests in any matter must receive adequate notice and have an opportunity to defend their interests before those can be legally altered or terminated.

In the dissenting opinion for Armstrong v. Manzo, Justice Harlan argued that due process was not violated in this case because Mr. Manzo had an opportunity to be heard at a meaningful time and in a meaningful manner when he appealed the adoption order. He believed that Texas law provided adequate procedural safeguards by allowing any interested party to contest an adoption within six months of its finalization, which Mr. Manzo did not do despite being aware of the proceedings. Furthermore, Justice Harlan pointed out that there were legitimate reasons why biological parents might not be notified before their parental rights are terminated - such as if they pose a threat to the child or have abandoned them - and these considerations should also factor into assessments of due process violations.