I replaced the rear center (hydractive) sphere on Rattiva this morning in my usual way, reused the seal, hand-tight and so on. The damm thing leaked

Replaced the seal. Still leaked although marginally less perhaps.
I wondered for a moment if I'd damaged the sphere block in using a large percussive instrument and a large punch to remove the old sphere.
But no, close inspection of the replacement sphere, an old accumulator that had lost a bit of puff, I found this:

That's paint on the sealing surface

Odd it never leaked in service as an accumulator...