Kinda what it says on the tin. I picked up an old generic lens for a few € at a flea market, and there are what look like silvery, reflective "splats" on one of the interior elements.
It looks like this, except there's a few of them:
I know it's a kind of separation (confirmed by the linked blog and a bit of screwdriver-turning), but my question is this: what causes this kind of separation, as opposed to the more "classical" effect of chromatic stripes? It seems rare - I had a hard time finding anything matching it.