Aberrations
The optical elements discussed above are not perfect. There are a number of aberrations that exist. Two of the most important are noted here. Your textbook gives a more complete description.
Spherical aberration
Rays off axis (non paraxial) are not focussed to the same point (for spherical mirrors and spherical lenses). This occurs since the approximations used in the calculations break down for larger angle incident rays. It is possible to use parabolic mirrors although they are more expensive to manufacture.
Chromatic aberration
Different wavelengths have different refractive indices and so are focussed at different positions. This can be overcome with compound lenses.