In NZ we use GIANT orange felt pens to mark our ballots, good luck hiding one anywhere. I've heard that pencils are sometimes used to prevent an attacker from

In NZ we use GIANT orange felt pens to mark our ballots, good luck hiding one anywhere. I've heard that pencils are sometimes used to prevent an attacker from substituting pens at the polling booth, with modified pens containing disappearing ink, but that seems like a problem better controlled by using new, easily identifiable pens each election. As for serialising the ballot-sheets, that enables a third party to check that a person they coerced into voting a certain way actually did as instructed, which undermines the value of a secret ballot.