Sounds like it must have been a copper oxide rectifier diode
:-)
or maybe a selenium rectifier.
(remember them?)
  __________________________________
|
|
| - Quantum Mechanics
|   The Dreams that Stuff are made of.
|
| -  All models are wrong -
|   some models are useful.
| _______________________________|

From: NCS Products <ncs@WORLDNET.ATT.NET>


>I built a 16F84 board using just a 4.7K diode from MCLR to
VCC (no
>blocking diode), and it in-circuit-programmed fine.
>