All plastics are not created equal. Some plastic trays, wraps and containers from store delis and restaurants are not designed for the microwave. The high temperatures that food reaches can melt or warp the plastic, and can cause the food to spill. Trays and containers provided with microwave meals should not become your office dishes. They may be convenient, but most trays and containers are intended for one-time use with certain kinds of foods.
Heating can cause margarine tubs, cottage cheese containers and foam meat trays to melt and warp.
Once again, check the directions. New technology allows some kinds of packaging to go from the freezer to the microwave. Use plastic wrap recommended for microwave use to keep moisture in and provide even cooking during re-warming. Placing plastic wrap loosely over bowls and dishes during heating helps prevent food from splattering and creating a mess. Remember that microwaving doesn't take a lot of time.