I don't think this is specifically PIC-related, so please feel free
to reply to me privately if you think the list wouldn't be interested
in your answer.

I was having a problem the other day where my 20MHz 16C63/JW seemed
to be running at more like 6-7MHz.  I wrote up a description of
the problem and almost submitted it to the list, but then I decided
to spend $0.75 of my own money buying another 20MHz crystal first.
Sure enough, the new crystal works fine.

I suspect and hope that the first crystal was just mislabelled.
Or is this something that could happen again?  Can damage to a
crystal change its frequency, rather than just break it?

Further details: I have no oscilloscope, but as far as my blinkenlights
programs showed, the first crystal worked steadily and reliably,
just at the wrong rate.  I have the OSC1/OSC2 capacitors installed
as recommended in the data book (22pF), and the OSC TYPE bits are
set to "HS" (assuming that the Picstart Plus software is doing its
job).  I don't think it's a marginal 16C63; I'm using two of them,
and in any case the new 20MHz crystal works with both.

Brian