Plastics - Some types of plastics can melt. They are not directly affected by the microwaves, but rather the heat of the food in contact with them is what does the damage.

Metals - Objects composed of metal in whole or in part can be put into the microwave, but they have to have certain qualities. For example, they cannot have sharp points and they have to be positioned within the oven cavity so as to have no arcing between the object and the cavity walls. In short, unless you know what you're doing, the best bet is to not put metal objects in the microwave.

Ceramics - Some types of ceramics that are used to make plates, mugs and similar articles are very good at absorbing microwaves. You can identify these articles by how hot they get in the oven even with nothing on or in them. They are dangerous because if you are unprepared, you can get badly burned.

Superheated water - This is an extremely rare phenomenon. It occurs most frequently in perfectly clean glass containers in which distilled water is put in the microwave to boil for several minutes. If there are no nucleation sites for water vapor to stick to, the temperature of the water rises, but it doesn't boil. When taken out of the microwave, it looks placid, but if a spoon or similar object is dropped into it, it will erupt in a spray of scalding water. Don't try this at home.

Flammable chemicals - The possibility of electrical arcing means microwaving any flammable material carries a risk of fire. A fire could cause the oven to fail in such a way that it blasts you with microwave radiation before it fails completely. And we don't want that.

Empty oven - Running the oven empty for an extended period - say several minutes - can put a lot of thermal stress on the magnetron (the vacuum tube inside microwave ovens that generates the microwaves) and it may fail prematurely. Newer ovens are made with magnetrons composed of a tough ceramic, but older ovens used glass magnetrons, and could fail very quickly when run empty. There would be some sort of load in the cavity, such as glass of water.

Live animals - Don't, because that's just wrong.

Explosives - Do I even have to tell you?

Other than that, enjoy!.