At this link you'll find a long essay ranting about the evils of microwave ovens. It claims that food cooked in a microwave, no matter what kind of food it is: "increases cholesterol, increases white blood cell numbers, decreases red blood cell numbers, and causes production of radiolytic compounds (compounds unknown in nature)". All sounds a bit dubious, but I'm actually more interested in one specific story told at the beginning of the article:

There was a lawsuit in 1991 in Oklahoma. A woman named Norma Levitt had hip surgery, but was killed by a simple blood transfusion when a nurse "warmed the blood for the transfusion in a microwave oven!" 

This seemed a bit unlikely to me, but a quick google search revealed that this same story, with almost the exact same wording, appears on many sites. So obviously this is a tale that the anti-microwave people have been spreading around. However, a little more searching reveals that the story isn't true. The incident did happen, but a jury found that Norma Levitt was killed by a blood clot, not by blood heated in a microwave.