Cracking or Breaking
A bowl that is not specifically designed for microwave use can crack or break due to the intense heat used in the process of cooking food. It is generally thought that glass or ceramic is safe to use in the microwave, but certain types of containers can shatter under the heat, causing shards of glass to end up in your dinner.
Plastic bowls can also break or melt, leaving pieces of the bowl embedded in your food. Tiny pieces of glass or plastic could end up in your meal, unknown to the person who prepared it.
Some plastic containers contain chemicals that, if ingested, could cause health problems to humans. These chemicals could leak into your food through the microwaving process without the bowl actually melting or breaking.
Chemicals are more likely to leach onto food that is high in fat. According to the Harvard Medical School's website, "when food is wrapped in plastic or placed in a plastic container and microwaved, substances used in manufacturing the plastic (plasticizers) may leak into the food.
In particular, fatty foods such as meats and cheeses cause a chemical called diethylhexyl adipate to leach out of the plastic".
Damage to the Microwave Oven
Using a container that is not microwave-safe could also cause severe damage to your microwave oven or even to your home. Some plastic materials will melt and stick to the inside of your microwave, making it harmful to use in the future.
Any container that has metal parts may cause sparks which could damage your microwave. Cloth material, for instance on a child's cup or cereal bowl, can catch on fire after prolonged time in the microwave.
It is important to know what materials you are putting into your microwave before attempting to use a bowl.