Food was not kept cold enough, hot enough, cooled fast enough, or cooked or reheated to the right temperature. Bacteria grow quickly in the "danger zone" between 41°F and 135°F, which makes this one of the most important food-safety checks.
Staff were not washing their hands when required, or a handwashing sink was blocked, missing soap or paper towels, or used for something else. Hands are one of the most common ways germs reach food.