I've monitored elections in Ireland and the Netherlands which both have an interesting problem. They each use Proportional Representation and ranked voting. Voters filling in their ballots mark each candidate with a number indicating their preference.
There are routinely 10+ candidates in each constituency. That makes it easy to encode a highly-likely to be unique voter signature in the lower order numbers, which would make it possible to sell your vote.
To combat this, when the votes are unfurled from the ballot boxes in public, tallymen (independent checkers of the vote) are expected to record only the first preferences, and if you start marking any more, you can expect a conversation with the returning officer. They are keeping an eye out.
If you are a scrutinizer of potentially invalid ballots, which I've done a few times, you're also not allowed to bring or take any notes.