Aberrations come in many forms and flavors. The aberrator can generate most of them and the links on the left will show you more . Focussing is needed to get your scope s focal point and your eyepieces focal-point at the right place. Due to misalignments Coma effects can be prominent in a telescope.
Stars appear like comets having tails on one side.
If you do not center the star used in startesting coma effects might be induced from your eyepiece. A well known aberration in human eyes but also in telescopes. Again misaligment can be part of the problem, but can be solved. Often asymetrical in the out-of focus images. Comes in two flavours, lower and higher spherical aberrations.