Leftovers should only be reheated once to adhere to food safety standards. Reheating food multiple times increases the chance of harmful bacterial build up, which can cause food borne illness. This is because reheating and cooling leftovers repeatedly increases the time that the food spends incubating bacteria at room temperature.
It is a myth that food can be reheated as many times as desired before it goes bad. Other food safety misconceptions include the "five second rule," as it is not actually safe to leave food on an unsafe surface for five seconds before consuming it. Another myth is that raw chicken needs to be rinsed before cooking, as bacteria is not killed by rinsing in water.