The Polar3D printer’s underlying polar coordinates architecture means that in order to perform a “straight-line” extrusion through the very center of the build plate, the printer must pause extruding when it reaches the center of the build plate and perform a 180° spin, before resuming printing. This makes this a point where if your printer’s calibration is less than perfect, it may be more apparent here (straight-line extrusion runs through the center point of the build plate) than anywhere else: with less than perfect calibration, you might see a jog in the output, a bit of a blob at the exact center, a “hole” in the object at the exact center, a slight bend in what is supposed to be a straight line, etc. (See Section 6.5: Printer calibration of the Polar3D Printer Guide for further details on Polar3D printer calibration.)
So if the appearance of an exact, smooth, straight line is an important part of the object you are printing, try to position the object so that (visible) such straight lines of material don’t have to go through the exact center of the build plate – or else be very careful with your callibration.