Coma is also produced by a Flat that has its vertical axis tilted towards the Main Mirror. This Flat will redirect the on-axis light cone from the Main Mirror as an off-axis light cone towards the Eyepiece, producing Coma Error.
Why is a Schmidt-Newtonian telescope free from Coma Error while a Pure Newtonian telescope is noted for Coma Error?  There are two reasons. First, the Flat in a Schmidt-Newtonian is fixed in the center of the Schmidt Corrector Plate where it is less likely to be jarred out of vertical allignment than the flat of a Pure Newtonian telescope.  Second, is the almost universal practice of incompletely figuring the Main Mirror from a spherical cross section to a parabolic cross section.
The universal method of Figuring a Spherical Mirror into a Parabolic Mirror is to polish a second smaller spherical section into the spherical mirror's central region. The two circular sections combined form an approximation of the true parabolic curve.