Astigmatism results in star images that focus to crosses rather than points.  Sometimes, as a photographer I have used this aberration deliberately for artistic effect. I would place an astigmatic filter in front of my lens and photograph street scenes at night. All the Street lights would appear as crosses because they were point sources of light being focused by an astigmatic lens.  However, in Astronomy, Astigmatism is most undesirable.
Astigmatism is caused by an asymmetrical mirror or lens. Not enough care was taken to ensure the grinding was symmetrical about the mirror's central axis. I am glad I mapped my mirror so frequently. I knew at all times how symmetrical my grinding was.

I was criticized by a visiting school inspector who expressed the opinion that I need not put so much care into the earlier stages of grinding, because the surfaces I was getting would only be ground away anyway. He was only partly right.  The experience I was gaining at curve control was invaluable.