The remarkable properties exhibited by metamaterials would be impossible without the complex scattering effects that occur as waves pass through the material and interact with its internal microstructure. For this reason, multiple scattering theory is crucial to our understanding of metamaterials. In this presentation, we will show how multiple scattering theory can break down in cases where the length scale of the internal microstructure is comparable to (or smaller than) the wavelength. This is not a flaw in any particular mathematical modelling technique; rather it is a fundamental property of solutions to partial differential equations. We will also sketch out some possible avenues for research into overcoming the problem.