Rome, Italy, Iphone5s, bought one year ago (christmas 2013), I have three cables, one the original from apple, two OEM. I use them in more or less the same settings : i have one in my bag, one near my bed, one usually attached to my computer, but they "rotate".
Recently (last 2/3 weeks) i started experiencing the problem, sometimes it does not charge, sometimes it makes intermittent charge/not charging, sometimes it say the cable is not compatible. Removing, flipping, rubbing them a bit, etc... and then they usually work.
It does this on all three cables, and all three cables show the black/bluish stains (call it whatever you like) on the fourth pin starting from the left, on both sides. Some smaller black stains appear also on the nearby pins in one side of two of the three cables.
I tried "cleaning" it, but no way. I don't have a microscope to see if it is oxidation over the pin or corrosion of the pin itself. However, bluish stains are often on the plastic part, black ones on the pin itself, and they seem to be different, the bluish ones can be removed by rubbing, the black ones no. If I try to scratch the pin with something solid (a needle for example), i can feel the surface of the affected pin has some concavity (holes, craters, whatever).
As to moisture, I think Apple (and most other manufacturers following Apple) are taking the wrong side here. We have often humidity around 80/90% here, and our phones are with us in what could be even worse situations, like crowded places, sudden termical changes (in and out of a car in summer, in and out of house in winter) that can cause condensation. So, they should either tell us not to carry the phones with us too often, or made them able to resist these quite normal "shocks". Unless you dipped your phone in your cappuccino, i would not accept the "moisture" argument if not as a clever excuse to refuse to replace your phone.
I will buy a new cable as soon as possible, because I want to check if it's my phone and if it's permanent (that is, if it will make also the new cable have the same problem in a short time), and also I'm afraid that, be it corrosion/erosion/whatever, the contact surface on the cable is now smaller that it originally was, and that can cause damage to the contact in the phone itself, especially if there is an hypotheses of some electrical arcing.