You have probably seen a lot of 24-105 lenses – the flex cable breaks after a certain time of using – and ALWAYS on the same place.
And on that position always appears a tiny broken line in the middle of the cable – never on the margin.
Today I looked at it again – and I have found, that on the other side of the cable there is on asymmetrical part, a piece of plastic. And when the cable rolls over that part, it is always pressed on the same sharp edge….
From now on I will carve it to a chamfered, or rounded form, to help the flex to roll smoother, without premature breaking.