Questions like this generally disturb me as I have visions of the poster or another reader attempting a "simple home experiment" with their household electricity. Are we all intelligent enough to "DON'T TRY THIS AT HOME" ?
When it comes to household electrics you should really assume that anything whatsoever inside a plug, other than it's normal contents of the bits of the plug and air, COULD cause a problem.
Getting more scientific I wouldn't be surprised if I tested 4 different brands of washing up liquid to find one or more quite conductive and also one or more quite a good insulator. It would all depend on the chemical composition, including, I suspect , particularly how much H2O the manufacturers have added/used in it. Those interested in doing experiments could safely test / measure resistance under low voltage conditions with a bog-standard home multimeter applied to a cupful of the liquid in question (No mains plugs or mains voltages involved !). I'd probably stick the probes of my 500V/1000V megger in it but I know what I am doing with these things.