Food
Leafy vegetables are a powerhouse of nutrients, fibre, and nitrates but shouldn't be reheated because these nitrates can turn into toxic variants and potentially cause cancer.
Eggs contain a lot of protein; if you reheat eggs, the protein becomes destabilized and causes digestion-related issues.
Despite being a favourite among all, potatoes should not be left at room temperature, as Clostridium botulinum can grow on the potatoes. It can only be killed at temp above 115 C.
Rice is a food that contains a harmful bacteria called Bacillus cereus, which survives even after rice is cooked. If not stored properly and reheated, it can cause food poisoning.
The repeatedly used cooking oils generate compounds like polycyclic aromatic hydrocarbons, which are carcinogenic.