Answer: They don't actually exist -- except in the minds of right-wingers. (Well, I'm not actually sure if Pat Robertson and the like believe in unicorns or the Tooth Fairy, but given the outlandish nature of some of their other beliefs, I wouldn't really be surprised.)
Chris Hayes breaks down the non-problem of voter fraud in a great post on The Nation's blog about how he "voter fraud" -- emphasis on the scare quotes -- is a problem that doesn't actually exist:
Every two years, Republicans gin up baseless accusations of "voter fraud," often directed at ACORN. The strategic imperative is simple: create a pretense that will allow them to more credibly hassle and hopefully suppress poor and minority voters.
Just to get this out of the way: in the real world, there is no such thing as voter fraud. There will be roughly as many fraudulent votes cast in this election as there were stockpiles of biological weapons in Iraq. That is to say, none.
Read the whole thing here.