Errors in High Resistance Measurements due leakage from nylon or PVC test leads

[Kiriakos GR]

Errors in High Resistance Measurements
When you are measuring large resistances, significant errors can occur due to insulation resistance and surface cleanliness.

You should take the necessary precautions to maintain a "clean" high–resistance system.
Test leads and fixtures are susceptible to leakage due to moisture absorption in insulating materials and
"dirty" surface films.

Nylon and PVC are relatively poor insulators (109 Ω) when compared to PTFE insulators (1013 Ω).
Leakage from nylon or PVC insulators can easily contribute a 0.1% error when measuring a 1 MΩ resistance in humid conditions.
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This is an interesting note which Keysight wrote at Operating and Service Guide / Truevolt Series DMM.

What is the teaching from it? 
If regular DMM they can suffer from measuring instability due to your choices of test cables and accessories, then just imagine of how worst the problem will become when using insulation resistance testers ...

In summary,
Errors in High Resistance Measurements? = Check your test leads quality and of what they are made of.