Unfortunately, mistakes can happen in even the strictest of labs. The infamous HeLa cells, which are extremely aggressive and fast growing, are known for being able to contaminate other cultures via aerosol droplets. A simple mislabeling of a flask could cause misidentification of the cell line in use. Not following stringent cell culture rules is also known to perpetuate the problem; sharing media and other lab supplies amongst multiple cell lines could risk contamination. Also, sharing cell lines between laboratories could be fuelling the problem – you may have unknowingly received a misidentified cell line.