I've been in retopo and have encountered errors when using Points and Quads tools. These error tips occur where no double edges exist. No amount of "Move" will discover a hidden face or edge. With P/F, no polygon is drawn but Quads are, in spite of error notice. However, some previously drawn polys are deleted. The workaround is to power on but this should be looked into. When it occurs in the Retopo process I cannot say. I encountered it in the torso and fingers. Also, use of the Symmetrical Copy tool at this point can delete polys and redraw vertices.