Having worked with specifying, manufacturing and testing gold contacts for many years, its a bit of a poor situation with the tips of the contacts being used to make electrical contact. Gold is a lot like a thick paste and pushes around and smears much easier than some metals, and the pressure is pretty high on the end of those contacts. Plating the ends of the contacts is also quite difficult, thats where the gold will always be the thinest, and where gaps or cracks in the plating are most likely to occur.
You do not want a gap or crack in the gold, since you then will get dry circuit corrosion with the nickel underplate. Dry circuit corrosion cannot be seen, because it it a thin electrical insulator for low voltages. The wiping action of installing a lens will normally wipe the film away.
There are tons of inventors who have created cleaning solutioons for electrical contacts, but most of them also wipe away some gold, and in the end, ruin the contacts. They beat a path to our door, but most don't understand the issue because they can demonstrate that it cleans, but we look at the surfaces under a scannng electron microscope and can see the damage. You can also see damage by cross sectioning a part, but gold is so soft and easy to smear that only the most talented and able technicians can section and polish a gold plated part successfully. We also use x-ray fluorescent test machines to measure for proper gold thickness, but they cannot see cracks or pores.
Generally, a wipe with a smooth clean cloth is well able to clean away any dry circuit film, and it should only be done sparingly. Non aprasive cleaners are ok, but they may get into the camera body and wreak havoc. Most of the time, just removing and remounting a lens fixes the issue. The issue could originally be a manufacturing defect, but if a eraser or contact cleaner has been used, that is also a suspect. Once the damage has been done, it will only get worse, so beware, even one time can mess things up.
