An old-style polyline might contain vertices that were not created explicitly; these auxiliary vertices were inserted automatically by the AutoCAD PEDIT command's Fit and Spline options.
You can safely ignore these additional vertices when stepping through a polyline with entnext because changes to these vertices will be discarded the next time the AutoCAD PEDIT command is used to fit or convert the polyline to a spline. The old-style polyline entity's group code 70 flags indicate whether the polyline has been curve-fit (bit value 2) or spline-fit (bit value 4). If neither bit is set, all the polyline's vertices are regular user-defined vertices.
However, if the curve-fit bit (2) is set, alternating vertices of the polyline have the bit value 1 set in their 70 group code to indicate that they were inserted by the curve-fitting process. If you use entmod to move the vertices of such a polyline with the intent of refitting the curve by means of the AutoCAD PEDIT command, ignore these vertices.
Likewise, if the old-style polyline entity's spline-fit flag bit (bit 4) is set, an assortment of vertices will be found—some with flag bit 1 (inserted by curve fitting if the AutoCAD SPLINESEGS system variable was negative), some with bit value 8 (inserted by spline fitting), and all others with bit value 16 (spline frame-control point). Here again, if you use entmod to move the vertices and you intend to refit the spline afterward, move only the control-point vertices.