Some time ago I used commercial wooden pot knobs. I never had a cracked knob, but discovered that other guitar makers had had problems at some time. This is the result of a typical breakage:
It happens when the grub screw is tightened beyond certain point, but I think that this is more common when the wood has some small defect that went unnoticed when the knob was made.
I decided to change this and make my own knobs, not just for this problem but also because this way I could make my own designs, stacking different woods for example. Initially I made them with a bakelite core, but later I moved to aluminum:
