Rice
You should always consume freshly cooked rice as it contains bacteria that does not stop growing even after the cooking has stopped.
Egg
Eggs should not be exposed to heat twice as it contains nitrogen. When heated, it can turn toxic.
Spinach
On heating spinach again, the nitrogen in it can turn into nitrites and that is toxic.
Cooking Oil
No cooking oil should be reheated as it can become rancid.
Mushrooms
Reheating mushrooms can cause digestive problems.
Chicken
The structure of protein changes after heating chicken and doing it frequently can cause digestive problems.