I just tried to program a 16c64 while it was in situ.  The chip appeared to
accept the program and it even verified at 5.5V but when I switched to 4.5V
the verify intermittently failed.  I kept thinking that my programmer wasn't
working.  After a few hours of looking at the hex listings, I finally thought
to remove the chip from the circuit.  Once I did this, the programmer work
flawlessly.  Finally, I determined that it was the oscillator that was messing
everything up.

DON'T apply an oscillator signal to the clkin pin when programming a pic chip!

I learned this the hard way.

Happy Thanks Giving,

Derrick Early (still a rookie)