A slightly different kind of creep happens in the silver-mica
capacitors in old radios. Many old radios from the 1950's had a small
silver-mica sheet in the base of each IF transformer. Each sheet had
two small capacitors deposited on it. One is in the plate circuit of
one tube, with 100 to 150 volts on both its terminals. The other
capacitor, about 1 cm away, is in the grid circuit of the next tube, at
near zero volts. Over the years many of these have shorted out from
one capacitor to the other. You can see the shorting is due to metal
creeping across the gap.

The shorting in this case is often intermittent-- as the 150 volts is
quite capable of zapping the thin metal film. If you ever hear a radio
that emits a constant crash of static, as if in a monsoon, this is
likely the reason.

I've never seen this happen in transistor radios, and I've played
hundreds of old ones. Is this problem limited to certain transistor
designs? Most US transistors went from alloy-junction type directly
to silicon planar, circa 1962.