PGD to PGC Crosstalk
While this is really another circuit constraint, this issue is so unintuitive, little known, poorly documented, but serious that it deserves its own section.
The standard Microchip cable unfortunately puts PGD and PGC on adjacent lines. Since this is a flat cable, this can and does lead to crosstalk between the two in some cases. For writing to the target, the programmer drives both lines. In that case a little low pass filtering can be applied by the programmer to soften the edges and reduce the coupled amplitude on one line from an edge on the other...