It's possible to put cobble into the facade slot of the painting machine, and then paint it with a facade, giving a block that renders incorrectly
I expect it to go into the other slot and paint the conduit facade, not be painted by it
Not available at time of writing
Removed unused Facade block
* re #3980
Facades are no longer valid paint sources (code complete)
Your suggestion to remove the ability to paint cobblestone was moved to the suggestion tracker.