For you systems people out there, here’s a simple feedback diagram that illustrates a general unsustainable system:

With increased production (of food, for example) consumption also increases. Increased consumption leads to population growth, which in turn increases production. This forms a positive feedback loop, limited only by 1) the availability of resources, or 2) the carrying capacity of the environment. A positive feedback does not mean a system will instantaneously collapse, but no unsustainable system can persist indefinitely.