The topic was covered some time ago on the PICLIST. One had a problem
when Vcc fall, and then rise again, but not fall to zero. Then both WDT
and reset hang and it was needed to power off the chip. Even external
reset did not work.

I think that the way to do it must be an external WDT to power of and on
the chip. It may cause problems with remembering data, but they may be
stored in the flash. Another had problems with similar behavior of the
PIC16F84 chip. It erased data in it's flash if the power was turned of
and up (slowly?). Maybee you need a checksum too?

Several years ago, I had a project with a LCD-display. It worked
perfect, but after days or weeks it hang. I needed to power off and
power up.
I decided to always make it possible to power up and off by software,
and e.g. verify the data on the display and do an automatic reset when
it hang, or to reset it automatic with some intervals.

Maybee a lot of chips has similar behavior if noise, or other transient
on Vcc occeurs?

Regards,
Jens Dyekjfr Madsen