Public speaking?
Fear of failure?
Difficult people?
For me, it would have to be handling difficult people.
The thought of an obstinate person being openly negative makes my head spin.
I tend to ignore or shut down when faced with such folk... But then a mentor laid some practical wisdom on me.
How often do we actually encounter someone that really needs to be addressed? Not very often.
In fact, almost never.
I still want to be ready in the off chance that someone else is having a bad day, or shouldn't be in the session at all. However, knowing my mentor was constantly around some pretty openly critical people or troublemakers all the time and he rarely had to result to extreme measures, like booting them out, made me feel better about jumping into running sessions with new folks.
Plus, I've found there's ways of identifying potentially difficult personalities early.
What's the scariest part of facilitation for you?