The photos show another case of dendrite formation, although this time it's not in a mild, low voltage scenario (like in the infamous ceramic filter case).
That dendrite formed on one of my car's spark plugs when the ceramic insulation cracked and leakage occured, making the engine misfire and hiccup.
Observe the fractal nature of the dendrite, reminding of a lightning (which is exactly the same phenomenon at a much larger scale).
After I cleaned the ceramic surface with an abrasive disk and applied a suitable silicone oil with very high dielectric strength to the tiny crack, the spark plug worked very well for a few days until it was replaced.
Moral: When dendrites grow
like a thorny bush,
your car starts going slow,
you may well have to push...
(Click on the photos to see a larger version, the initial magnification was 25X and 200X, respectively).