* I remember an identical problem occurring a limited number of times recently. I think this happened exclusively when checking fragments of a longer take. It seems to result from:
1 using just a subset of the currently active take (squashing the grey slider at the bottom)
2 maybe, switching the fps rate for testing
3 possibly, there is a connection between the length of the active take and the fps rate.
* Presumably, there is some algebraic misfit where the rulers are misaligned by a pixel or two on the redraw of the GUI.
* Reentering 0 / zero in the frame counter (or delete plus tab) seems to force a redraw for the time line. There may be other fixes. In any case, C3D still renders the invisible frame. Of course, it is irritating if you wanted to tweak some keys at frame 0. Oops, looks like you can edit keyframe #0 (via parameters), even when it is not visible.