Molecular diagnostics is referred to as the detection of genomic variants, aiming to facilitate detection, diagnosis, subclassification, prognosis, and monitoring response to therapy. Molecular diagnostics is the outcome of the fruitful interplay among laboratory medicine, genomics knowledge, and technology in the field of molecular genetics, especially with significant discoveries in the field of molecular genomic technologies. All these factors contribute to the identification and fine characterization of the genetic basis of inherited diseases which, in turn, is vital for the accurate provision of diagnosis. High-throughput methods, such as next-generation sequencing or genome-wide association studies, provide invaluable insights into the mechanisms of disease, and genomic biomarkers allow physicians to not only assess disease predisposition but also to design and implement accurate diagnostic methods and to individualize therapeutic treatment modalities. Molecular diagnostics has undergone a period of rapid development and growth in the last decade. The implementation of new high complexity tests and integration of new technologies into the clinical molecular diagnostics laboratory has been critical towards advancement to the goal of achieving precision medicine. Molecular diagnostics encompasses diverse fields such as infectious disease, genetics, pharmacogenomics, and oncology, however, the underlying principles and potential sources of error are common among these different applications. Common errors that may occur with diagnostic molecular assays are discussed here. As with other sections of the clinical laboratory, a strong program of quality control and quality assurance is necessary for the detection of problems, to monitor errors, and to implement methods to assure quality.