So I got an old lens on eBay. It was advertised as 'clean and clear'. But when I received it, look what I found:
Disgusting fungus. Just looking at it makes my skin crawl. I don't dare get it anywhere close to my cameras or other lenses. Hopefully I can get this lens returned without issues.
Note: I can only see the fungus at certain angles. To get a photo of the fungus, I had to light the lens with direct flash. Here is a shot where the fungus is not at all apparent. Caveat emptor.
How I Keep Fungus Out