High-protein foods such as eggs contain a lot of nitrogen. Reheating could cause this nitrogen to oxidise, which can be carcinogenic.
By reheating beetroot dishes, you run the risk of ingesting a dose of toxins instead of all the wonderful advantages of this vegetable.
Nitrates are in abundance in spinach, and when they are heated to a high temperature, they can transform into nitrosamines, the majority of which are carcinogenic.
Long-term room-temperature storage of cooked potatoes can result in the growth of clostridium botulinum bacteria. Eating food contaminated with these bacteria can have very serious implications.
Mushroom proteins can be harmed by enzymes and bacteria start growing at room temperature if they are not stored appropriately. Reheating mushrooms can accelerate this growth and give you an upset stomach.