1. Anything that's made from metal e.g. metal containers, canned food, and especially aluminium foil
Metal reacts to heat AND reflects microwave energy, which could possibly cause irreparable damage to your microwave. So, if you're planning on heating up your leftovers, remove the aluminium foil covering it before putting it in the microwave. Otherwise, sparks will fly and your microwave could be on fire before you realise what's happening.
2. Plastic bags and disposable paper bags
According to the USDA, they are "not sanitary, may cause a fire, and can emit toxic fumes. Intense heat may cause a bag to ignite, causing a fire in the oven. The ink, glue, and recycled materials in paper bags can emit toxic fumes when they are exposed to heat. Instead, use purchased oven cooking bags."lifehacker.com
3. Stainless steel travel mugs
The stainless steel blocks heat from warming up the liquid (which defeats the purpose) and can even damage your microwave.
These little blobs will catch fire and quite possibly go KA-BLAM in the microwave.
Not only will they start smoking, raisins can send off enough plasma energy to damage your microwave.
Don’t try to boil a mug of water in the microwave. The water superheats but is unable to physically boil because the vessel remains cool, so when you try to stir it or add a teabag it boils all at once, exploding upwards, right in the direction of your face. If you must microwave water, stick a stirrer or something in it so it can boil as usual.thedailymeal.com
7. Anything made of plastic, especially one-time storage containers such as margerine tubs, styrofoam boxes, and yoghurt containers.
Unless they are labelled "microwave safe", you should never microwave anything housed within a plastic container as most, especially the ones manufactured for one-time use, are not made to withstand extreme heat. The containers could warp or melt in the microwave and release harmful chemicals into your food.
8. Dried chilli peppers, especially cili padi
9. Sauces and soup without a cover or a lid
10. Mobile phones and smartphones. D'uh.
If you turn the microwave on with nothing in it you’ll possibly blow the whole thing. Since there is nothing to absorb the microwaves, like food, the magnetron - the thing that makes the microwave function - ends up absorbing the microwaves and essentially self destructs.news.com.au
Make sure your mugs are not made of metal or plastic if you're gonna attempt any of these delicious, easy-to-make treats: