What Does X on Stability Circles Mean

Sometimes there is an X drawn for a stability circle, what does this mean?

When the radius of the stability circle is very small, an X will be displayed instead of a data marker and circle.

If you zoom in close around the X, you can trace the outline of the circle with the cursor by starting at the middle of the X and moving outward. The cursor will stop at the radius, move the cursor around the center of the X and the cursor will trace the radius.