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SunTour indexed shifting cable housing. It was parked outside in the sun for years. Shifting worked when the bike was parked, but on returning at a later date, applying cable tension (to change gears) caused the wire spiral to pull out from the plastic housing. Once free of the housing, the strands of the housing separated and the cable pulled through. The bike was ridden with the derailleur limit stop readjusted.
An important observation here is that the plastic housing is a structural element in shift cables, and that plastics can be damaged, leading to failure, where metal survives intact. Note also that brake cables are made using a single spiral of wire in which the plastic jacket is not a structural element. The difference means that brake cables do not suffer failure if the plastic sheath is damaged, but it is vitally important to not use an indexed shifting cable for brakes, as it can lead to sudden brake failure when the brakes are being applied the hardest, which is often when a failure is the most dangerous.
See also FAIL-044.html.