Quote Originally Posted by Bruce
Yes the clock & data pins definitely matter if you program in-circuit. A load on the clock pin in particular will really mess with timing.

Read Microchips' DS91017B ICSP guide for a detailed explanation.
I had the same issue rear it's ugly head some months back. If your code is evolving, you really should keep MCLR as a reset. The last programming action will be to flip the config fuse for the MCLR pin to become an input. If you place a 1 K resistor between the clock pin and the circuit it is attached to, this will help. Same with Data pin. For ICSP, I raise the pullup resistor to MCLR to 22K. It helps when your power supply is 2 to 3 volts and the programming power is 5 volts. If that's not enough, a schottky diode from B+ to the VDD pin will allow ICSP in a circuit that can't take 5 volts. I fried come 3 volt comm chips this way before the stupid light went off!
I perconally traded the 12F series for the 16F690 in the 4 mm package. 18 GPIOs and 4 K of flash memory!Boy is this living!