The club body and handle are manufactured by Henrys 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 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, so you can keep on juggling!
Cracks in the handle seams are covered under warranty, but you have to pay for shipping, and the handles are only $5 each, so you might prefer to simply order replacement handles.
For information on how to swap the club body parts, check out our support page here: https://flowtoys2.freshdesk.com/support/solutions/articles/6000214589-vision-clubs-swapping-out-club-bodies-handles-knobs-caps