When exactly does weight loss become dangerous?

So, I am about 50% of the way to being the normal weight for my size. (depending on what system you use)

I still have a couple of pockets of fat that I need to get rid of, but they will soon go. Once they do, I plan to continue the fat loss while building more muscle.

My idea is to have very little bodyfat so that I’m almost scary to look at. I have heard of some bodybuilders getting their bodyfat very low, but I am a little concerned.

When exactly does weight loss become dangerous?

At the moment, I am losing fat by the day, visually noticable changes every 2–3 days.

If I decide tomorrow, that I’m going to double my training period, do I have to double my calorie intake?

My body still has plenty of fat, but what will happen if I start burining it much faster than now? even though it’s fast now?

As long as I eat 2000 calories a day will I be safe? even if I start burning 4000 a day?

What are the major risks of weight loss and most common things people do wrong?