There will always be the possibility of things going wrong in the course of a symposium. As a symposium organizer, you have to accept this reality. Then, after accepting the harsh reality, you need to figure out ways of minimizing that probability of things going wrong. You can’t eliminate the possibility completely, but you can definitely lower the probability: and that is what you should be aspiring to do.
The first way in which you can minimize the probability of things going wrong in the course of a symposium is by having a plan B for every contingency. If, for instance, any one of the symposium speakers fails to show up, you need to have something to occupy the entire duration of that symposium plenary session.
The second way in which you can minimize the probability of things going wrong in the course of a symposium is by working with reliable service providers. You should ensure that the service providers you work with are people who truly have a track record for reliability. You should also work with service providers who have reputations: service providers who will do everything to protect their reputations. You should subject the service providers you consider working with to a very stringent vetting criteria: akin to the one that is used by the correctional departments to vet jail service providers. That is where, for instance, the people who supply various materials and services to jails are subjected to extremely thorough vetting before being given the contracts. That example may be extreme, but it demonstrates the degree of care you should take, when selecting service providers to work with in preparation for a symposium.
The third way in which you can minimize the probability of things going wrong in the course of a symposium is by making preparations for the symposium in good time. In most cases, things go wrong when they are done hurriedly. If you delay your symposium preparations to the very last moment, you will end up having to do things hurriedly. And this will greatly increase the probability of things going wrong in the symposium.