Dry skin is a reasonably effective insulator against the kinds of voltages typically produced by wall warts, i.e. less than about 24V.
But wet skin, especially if there's salt involved, is considerably more conductive. As a kid I had applied a mere 12V to some slightly salty water, when some small object fell into the soup. Thinking "it's just 12V" and thinking it harmless, I went to fish the offending object out with my fingers. This was a rather unforgettably unpleasant experience, I will attest.
So, cautionary tale there - even with low voltage, watch out if your hands are wet!
Mostly the potato, I'd say... as it will probably contain potentially toxic copper salts after the operation. If the adapter generates 9-12V, it will be pretty unpleasant to have that voltage in your mouth, but probably not dangerous (like licking a 9V battery to check if it's still charged...)
