The club body and handle are manufactured by Hnerys using an extrusion blow molding technique that leave small seams near the ends - these can vary in length and width and are sometimes mistaken for cracks. 

It is possible for the seams near the knob to split open in some extreme cases - you would be able to see the shiny clear tube inside if this happens. This is primarily an aesthetic concern - it does not spread very far or cause the handle to fall off or otherwise effect juggling performance, so keep on juggling!