hm, I've heard that it's unhealthy to loose weight too fast, but I haven't heard of it causing permanent damage...

My best guess? Organ damage.
depending on your initial weight and fitness, you can really only loose so much fat a week. For an average person, that's probably only around one or two pounds.
If you're loosing more, it means you're loosing water weight, muscle and whatever else your body burns instead of fat. Your body really really needs those things, and I imagine it'd cause organ damage if you dehydrate yourself for too long.
Plus, many of the diets that promise fast weight loss are ones that cut out one whole food group. Like, no carbs. Sounds good, but that's bound to cause an unhealthy and possibly permanently damaging imbalance. On the long run, you need carbs to live.

also; obese people are said to get loose skin if they loose too fast.. and the only other thing I can think of is.... um being underweight is unhealthy?

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