Do you mean just being scared because you've just become lucid or the content of the dream being scary, such as monsters etc?

I think both qualify really, as in my first LD I was falling from the sky and didn't know why, but somehow parts of it were so realistic that I kept doing a double check just in case it was real lol.

Mostly, I think if you're new to LDing in general then it could be scary just getting the lucid feeling, but first time it'll be a shock, followed by..hey cool, I'm dreaming! followed by excitement and then waking up wanting more.