>>
           I am currently contemplating the risk of damage to my humble
       16C71JW. Basically I am powering it with 5V, but I have the o/p
       of a dual rail OP AMP going to one of the ADC inputs. The OP AMP
       is capable of swinging to +/- 9V and I am therefore worried. I
       have put a 1K series resistance prior to the pin and a 5V6 Zener
       from the pin to earth. However, this is a bit hit and miss and I
       would love to be told the proper way to protect the i/p by some
       experts.
             Yours hoping for a technical slapping!!
<<

I personally don't usually bother with zeners on PIC inputs, given the
existence of clamp diodes; a zener between VDD and ground may be a good
idea, though, since PICs generally don't like excessive VDD's.

The 1K resistor's probably a pretty good call; I had a 1K between the
tap on a pot (0-5V) and the PIC, and found it to be adequate even when
a ground loop put 120VAC on that pot; the 1K resistor blew open, but
the PIC was undamaged.  When people say PICs are indestructible, believe
'em.  The following is a rough catalog of non-OTP PIC's I've destroyed:

16C84 -- Accidentally connected RB6 to about 90VDC @unlimitted current.
  Remainder of chip worked fine, but couldn't reprogram without that
  pin.

16C622's (a few) -- Eventually wore out due to problem in programming
  software (the prog. pulses were sometimes too long).

12C508's -- Some OTP's of these failed for the same reason as the 622's
  above.

17C756 -- Mis-inserted into hand-built programming fixture (no current-
  limit on that one).  One of the bonding wires was sorta glowing (it
  was a window part--OUCH!)

16C925 -- Applied VPP without VDD and tried to program (was ICSP'ing and
  didn't have enough juice on my prog'ger to power VDD).

Plus probably a half dozen or so PICs of various types whose legs gave
out from too many insertion/removal cycles.  If you look through the
list, only one failure was due to something other than a programming or
handling failure, and that's going through a LOT of chips.