Would explain why they stop charging then...
It's only been in the past 12-18 months that they've started doing it, which makes me think it might be my wife's iPhone 5S - would fit the time frame approximately. I never noticed it before then with our other lightning devices. Have spoken to her and we're going to try only charge her phone with the same cable and see what happens.
In order - 2 Apple cables, then the 2 new ones from Kmart that were purchased around Xmas. I didn't take a reverse shot, but they are the same - 4th pins, black. The 2 Apple ones are dead - wont talk/charge - and I've chucked out 1-2 (more?) other Apple cables with the same issue. The white Kmart cable still works if you connect it the "right" way, but tends to "bounce" a bit first til it finds a good spot. The silver cable still works - but you can see the corrosion (?) starting.
Someone said to try rubbing alcohol... so I've just given 1 of them a rub with some Iso. It does look like it cleaned it up a little bit.... but only minor change. Worried I'll destroy it - but then, it already is failing to work.