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> I also just noticed a pretty obvious thing that was causing
> problems:  if we let the internal protection diodes of the chip
> kick in, that will create a low impedance path to Vdd or Vss,
> and thus may cause problems if one expects the PIC input
> to behave as high impedance

Don't rely on the internal diodes, the whole chip may latch up.
If it does you can probably kiss at least that pin goodbye. I've
had it happen. 12V signal (through 1k with a 4V7 zener to 0V)
had noise on it that went below ground to the tune of ~0.8V. The
PIC locked up and fried. I added a BAT46 Schottky diode and
10n cap to 0V, no more problems

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