Greg,
I now have a better understanding of what you have discovered. It probably is a program bug, but, regardless, it should be impossible. I was able to observe it in a dry run (router off) at a very slow feed rate. It is indeed scary, and may help to explain a few of the strange (but rare) behaviors that I have experienced with some of my older gcode routines that I switched from a DOS driver to Mach3.
Perhaps Art or Brian will pick up on this thread and work to help resolve, or at least explain, this issue.
Thank You,
John Champlain