Allowing food to stay in the temperature danger zone long enough for harmful bacteria to grow to unsafe levels.
It is letting potentially hazardous food stay in the danger zone of 5 degrees C to 60 degrees C long enough for harmful bacteria to grow to unsafe levels.
Usually not. Most food-poisoning bacteria do not change how food looks, smells or tastes, so you must rely on time and temperature records, not your senses.
Not always. Some bacteria produce heat-stable toxins that reheating cannot destroy, so food that was badly abused should be discarded rather than reheated.
Keep cold food at or below 5 degrees C, hot food at or above 60 degrees C, cool food quickly, avoid overloading fridges, and monitor temperatures so issues are caught early.
Assess the total time in the danger zone. For ready-to-eat food apply the 2-hour / 4-hour rule, record the decision, and discard food that has been in the zone too long.