Thank you for posting your feedback. We're sorry to see our low star review on DAP Phenopatch All-Purpose Spackling Paste. While this product has a biocide added at the time of manufacture, biological contamination (mold/mildew) is still possible in almost any water-based product, given certain factors, such as the age of the product, storage conditions and environmental exposure. Given that the contamination (presumed to be mold/mildew) you observed in your product was not present upon initially opening the sealed package and was only observed after the initial application/use, it would tend to indicate that contamination (exposure to the biological contaminant) occurred post-production/after first use. At your convenience, please contact DAP’s Technical Customer Service Department at 888-327-8477 so that we can obtain some additional information from you. When calling in, please have the product package available, as we will need to collect production information from it.