[Targalid]

Because you live in California and not in the salt capitol of the world, on the east coast, I suspect that on this single lead that you had some contamination. This is what we see happen to our cars where the roads are salted in other parts of the US. You had the unfortunate experience, though a minor one, or having this happen to a single contact on your distributor cap. Any humidity in combination with salt accelerates corrosion of ferrous metals via a process called chelation. It also isn't too good for most other metals as far as that goes. I suspect that when the cap was installed, the person who did so perhaps had some salt on his or her fingers (potato chips plus beer = better mechanic experience) and touched this single contact point. As the previous poster says, use some dielectric grease next time and it will help minimize this occurrence. Most caps and wire sets come with a small packet of grease for this purpose.