What can go wrong?
It could be as simple as your flash not working. Or your CF card failed you. Or you don't have enough CF cards. Or your lens is not long enough. Or your 3 weeks old DSLR suddenly stop working. Or the CF card you brought home after the shoot doesn't have any images in there. Or you don't have enough batteries for the flash or the body. Or the lens AF mechanism failed, and you are lousy with MF. It can be other people interfering with your shoot. People in the way doesn't mean you don't need to deliver. What happens when people get in your way? What happens if you cannot tell this people off or push them away? Predict and pre-empt this situations. Talk to other people with experience. Please add on potential screw ups from here.