That's a nasty little problem that thin mirrors can fall prey to as well as mirrors that don't get turned enough as you make them.
Basically, the problem is that there is a different FL as you rotate the mirror on the Foucault test. You can see it as a tilted shadow as the KE is moved into the beam of light being returned. Easier to see tho is to use the Ronchi Test and see if the line in the middle is straight or not as you rotate the mirror.
With fairly thick mirrors and a good rotation program while you are polishing and figuring, you won't have a problem with astigmitism. I've only seen one really astigmatic mirror and that was at RTMC.