Parabolic mirrors can focus light tightly, a spherical mirror produces aberrations particularly along the outer edge.
Coma is most often encountered in holography when a parabolic collimation mirror is used off-axis.
When the lens is more oblong than it is spherical. It's also the most common vision problem with the human eye.
If a lens diffracts differnt colors in different ways, then you get chromatic abberation. The classical example is a prism that breaks white light into a rainbow of its components (ROYGBIV). Mirrors are imune from chromatic aberrations.