I actually thought of this one while misreading the name of another topic, and thought it would be interesting to discuss. I have several of these, actually:
1. Being forced to choose between my own life, and that of someone I cared about.
2. Being surrounded while being publicly ridiculed and mocked, and being forced to endure it because there is no escape.
3. Having to watch helplessly as something or someone I care about is destroyed (I think it's worse if you have to watch).
4. Becoming so invested in something that I can't extricate myself if something starts to go wrong, and thus be forced to bear all the consequences of the failure.
5. Being trapped forever in a place with no computers, books, telephones, or other people.
There are probably more, but I can't think of them just now. So, do you have any? What situation do you fear most?