On Shimano (external routed shift cable), I inspect the right side cable (rear) regularly for fraying. I've had two of these break on me. The breakage is always preceded by a degradation in shifting precision.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter