Be careful when putting plastic covered food in the microwave, you could be creating a health hazard!
Make sure you do your research on plastics and follow directions closely. It's not good enough to presume an entire category of plastics is safe just because one product in the category is safe. For example, frozen meals are often packaged in microwavable trays made of PET, but not all PET plastics are the same. Some could warp if subjected to high heat. Also, in regards to frozen meals, it's important to note that since food heats more quickly than plastic, it is equally important to ensure your food does not come into direct contact with a plastic cover used to contain moisture while in a microwave oven, unless you are assured it is safe to do so. It is recommended that you leave at least on inch between the food and the wrap covering the dish (regardless of the container) to prevent your plastic wrap from melting onto your food
The industry's advice to consumers is to always play it safe by only using packaging and wrapping that is either labeled for microwave use, or includes microwave cooking instructions.