I tend to keep fast food coffee cups (McDonalds and BK) for a while and use them for their intended purpose, ie drinking coffee, since I dislike ceramic mugs. Is there any reason why I shouldn’t microwave the cups if my coffee gets cold?
The one time I tried it, the foam cup melted.
This, which is pretty much the only risk associated with microwaving food containers, aside from possible flash boiling if you overheat it. For other microwave myths, here is a nice summary (PDF) of the supposed dangers (some people say that plastic leaches carcinogens but that isn’t true; some plastics do produce dioxins when burned in a fire, but not if simply heated).