The presence of a foreign substance on or within a polymer often affects the mechanical, chemical and thermal properties of the material. The change in strength and rigidity of a polymer resulting from the plasticizing action of a sorbed chemical or due to the withdrawal of an added plasticizer by the leaching operation can seriously affect the useful life of the material.In the real engineering world, incompatible chemicals and lubricants get onto various plastic components unexpectedly through design, manufacturing processes, customers services and repairs. This paper presents a number of case-studies which illustrate how undesirable chemicals found on plastic parts can affect product performance and cause damage to the parts. They include: 1) plasticizer from PVC strip 2) mold-releases used to remove the part or excess foam 3) oily finishes used on fasteners 4) metal stamping fluids 5) rust inhibitors used on metal stampings 6) lubricants used for ease of installation 7) cleaning solvents and lubricants used in customer services. The compilation of these studies will give engineers and customers an awareness of the effects of chemicals on plastics. As a good engineering practice, all chemicals that come into contact with plastics components should be checked for compatibility before they are used. This will reduce unexpected product failure and extend the life of plastic parts.