Cooking a hard-boiled egg in the microwave, whether shelled or not, results in an explosive buildup of steam, much like a little pressure cooker.
A bottle of breast milk can be heated unevenly by a microwave, causing "hot patches" that can severely burn a baby's mouth and throat if they aren't handled properly.
Processed meats frequently contain chemicals and preservatives that help them last longer. Unfortunately, microwaving them can make them harmful to your health.
Rice has a problem with Bacillus cereus, a resistant microbe. This bacteria can create deadly spores when heated.
Since microwaves do not thoroughly or uniformly cook all portions of the meat, pathogens such as salmonella are more likely to survive.
Naturally occurring nitrates, which are highly healthy on their own, may transform to carcinogenic nitrosamines when heated in the microwave.
The same chemical transformation occurs when reheating nitrate-rich beets and turnips as when warming spinach. Fortunately, they are equally delightful when cold.
When hot peppers are reheated in the microwave, their spicy capsaicin is released. Airborne chemical can burn eyes and throat.
Reheating potatoes is risky. Cooking potatoes in aluminum foil shields C. botulinum from heat, so it can still grow at room temperature and cause botulism.