When reaching the limit, Mach3Mill and Mach3Turn should generate a coherent error message. I was just told, in a different part of the forum, that reaching the limit causes the bizarre behavior in the G-code list window.
And the documentation should spell out the demo-version limitations, perhaps in the installation section. That's the first place I went looking for it.
Apparently Mach3Turn has a lower limit than Mach3Mill. I was expecting a 500 line limit, based on what I'd read, and I wasted a lot of time finding and trying to figure out the bizarre behavior I was seeing.