When can you NOT waive your rights?

Article 6 of the New Civil Code of the Philippines provides:

Article 6. Rights may be waived, unless the waiver is contrary to law, public order, public policy, morals, or good customs, or prejudicial to a third person with a right recognized by law. (4a)

In Pampanga Sugar Development, Co., Inc., v. Court of Industrial Relations, et al.,[1] the Supreme Court reminded the parties that while rights may be waived, the waiver must not be contrary to law, public policy, morals, or good customs; or prejudicial to a third person with a right recognized by law.[22] 

[1] 200 Phil. 204, 213 (1982).