Leftover  rice can harbour spores of bacteria that survive the initial cooking process, according to shared United Kingdom's National Health Survey. When reheated improperly, these spores can multiply and cause food poisoning. 

So if you do reheat rice, ensure it's heated thoroughly and promptly.

Chicken

Reheating chicken, especially if not done at the right temperature, can lead to the growth of harmful bacteria. It's crucial to reheat chicken thoroughly, ensuring it reaches an internal temperature of 165°F (74°C).

Spinach and Other Leafy Greens

Leafy greens, when reheated, can lose their nutritional value due to the breakdown of nitrates into nitrites, potentially harmful compounds, according to a study published in ScienceDirect. It's best to consume these greens fresh or use them immediately after cooking.

Potatoes

When potatoes are cooked and then left at room temperature for too long, they can harbour harmful bacteria. Reheating potatoes can cause a multiplication of these bacteria, leading to food poisoning.

Eggs

Reheating  eggs can lead to a change in their protein structure and cause digestive issues. If you need to reheat eggs, do so at a low temperature and consume them immediately.

How To Reheat Food Safely

According to the Centre For Food Safety, here's how you can ensure that your leftovers are reheated properly:

• Use a food thermometer to measure the core temperature of the food.
• Reheat the food on high heat. 
• Check reheating time.
• Reheat the food till it boils.

While reheating food is convenient, certain foods are better off consumed fresh or with immediate reheating precautions to avoid health risks. It's important to handle and reheat food properly to minimize the risk of foodborne illnesses.