Studies of the same cell lines by different laboratories are common in the literature and often show different results with the same methodology. Use of best cell culture practices is essential to ensure consistent and reproducible results. Assay outcomes are easily influenced by many factors including changes in functionality, morphology, doubling time of cells, passage numbers, microbial contamination, and misidentification of cells. Simple observation, monitoring, and documentation of cell morphology and behavior, including growth rates, provide early warning and should be standard practice. Changes may indicate microbial contamination, genotypic drift due to high passage number, or cross-contamination with another cell line. Rapid molecular methods allow the identification of microbial and cross-contamination. Increasingly, authentication of cell lines is a prerequisite for scientific publication to avoid erroneous results entering the literature.