Polymer nanocomposites are promising materials for various applications in the electronics, biomedicine, and aerospace industries. However, the fabrication errors or defects, e.g., induced porosity, significantly impact the performance and the reliability of devices fabricated from polymer nanocomposites. Hence, this study has comprehensively carried out an investigation into the effects of defects on the properties of photovoltaic active cells, biomimetic scaffold, and aircraft structures that are fabricated by using polymer nanocomposites. Agglomeration is another defect that degrades the intended properties of polymer nanocomposite devices. For photovoltaic devices, defects can be controlled by the selective modification of organic semiconductor molecular structures. In addition, proper optimization of the process parameters and the material selection, are effective approaches for obtaining excellent photovoltaic cells. Furthermore, the presence of impurities, a non-homogeneous mixture of organic and inorganic materials, and the incomplete solubility of nanoparticles, are detrimental factors that affect the cell proliferation performance of biomimetic scaffolds. These technological imperfections must be also avoided when producing parts for aircraft structures. In other words, impurities introduced during the synthesis or processing stages can lead to irregularities in the material structure, which often affect its mechanical, electrical, biomedical, and optical properties. The understanding and mitigating of these factors are crucial for the optimization of the properties and performance of polymer nanocomposites in various applications.