Do plastic containers become toxic when you put them in the microwave?
There is some truth in it as not all plastics are equal. Here's the advice from the US Department of Agriculture Food Safety and Inspection Service about the use of plastics in microwaves:
"Only use cookware that is specially manufactured for use in the microwave oven. Glass, ceramic containers, and all plastics should be labelled for microwave oven use.
"Plastic storage containers such as margarine tubs, take-out containers and other one-time-use containers should not be used in microwave ovens. These containers can warp or melt, possibly causing harmful chemicals to migrate into the food.
"Microwave plastic wraps, wax paper, cooking bags, parchment paper, and white microwave-safe paper towels should be safe to use. Do not let plastic wrap touch foods during microwaving (to prevent the wrap from melting). Never use thin plastic storage bags, brown paper or plastic grocery bags, newspapers, or aluminium foil in the microwave oven."
The bottom line: Partly true
Containers that are not designed to be microwaved haven't been tested for this use and can be unsafe. Glass, ceramics and plastics labelled as microwave-safe are the best bet.