One thing that is sometimes mistaken for ESD is voltage breakdown in
components. An example would be pic pin driving a transistor driving a
load(for this example imagine a common-emitter with some base resistor). If
the transistor is overvoltaged(due to inductive effects, perhaps)
on turnoff, the pic is going to get crap sent back in on the pin it is
outputting from, and also the ground is going to get yanked around by the
emitter of the transistor(sometimes very significant!).

-Bob