In hunt to get inexpensive sharp telephoto prime lens, I bought Sigma 400mm f/5.6 APO for $255 from ebayer with 100% positive feedback.
Lens arrived well packaged, perfect looking externally. Main optical element clean. I made couple test shots that seemed slightly soft.

When I looked inside - what a bad surprise. Looks like a dens web.

Here are images that I made using Tamron 90mm f/2.8 1:1 macro.

I already contacted seller, notifying about problem. But don't know yet what he will answer me after seeing the images.
I was reading on web about Lens Fungus and found one poster mentioning that lenses are most affected if stored in leather cases. Mold is also fungus, by the way.

This lens came in nice black leather case.

Fungus is feeding on organic matter (dead wood, dry bread, other carbohydrates). I don't believe that lens byitself is good food, but if leather case is contaminated - spores will get inside of lens and will try to grow - web like.

On one of my images, if looked at "Original" multiple grain-like spores are visible with sprouts starting from them.

Bottom line:
Don't use leather cases for lens storage. Don't keep any food in same bag/backpack, even in different section of same bag.

Stan