There's a flat disc behind the rotor on the distributor. This has a rubber seal on it.
These break down and spark escapes (grounds) through to the block.

A smallish parts that seems to be rather expensive but should be replaced when a distributor cap is also replaced.

I've seen some cars where this disc has never been changed. Owners have been frustrated by misfires, have changed leads, plugs, dizzy cap and rotor but never changed the disc - and the disc ended up being the source of the misfire.