Ignoring the Dispersive Properties of Materials
You may have carefully chosen your optical materials based on refractive index, durability, and a host of other important parameters, but if you ignore dispersive properties you could run into trouble.
Often we imagine light as a single ray, always maintaining its own integrity even while manipulated by lenses, mirrors, and other optical devices. It turns out that concept of light useful though it might be in its simplicity isn’t always a very good visual for what is going on. Different wavelengths of light behave differently and propagate at different speeds. The index of refraction itself changes as wavelength changes. That makes a good knowledge of the dispersive properties of optical materials very important if you don’t want to run into unpleasant surprises.
Ignoring the dispersive properties of materials may simplify calculations, but it blindsides you to some very important features of light.