Studies of ballots have traditionally focused on roll-off, candidate order, and partisan advantage. This study is among the first to assess the impact of ballots on individual-level voter errors. We develop new hypotheses by bringing together theoretical insights from usability research and political science about the effects of ballots with and without a straight-party voting option. By comparing voters’ intentions to the votes they cast, we are able to create two measures of voter errors: votes unintentionally cast for the wrong candidate and unintentional undervotes. Voters generally make fewer errors of both types when using a standard office-bloc ballot than when using an office-bloc ballot with a straight-party option, with the number of wrong-candidate errors substantially exceeding the number of unintentional undervotes. Voters’ background characteristics have a significant impact on their ability to vote without error. Our results offer a new perspective for evaluating the use of the straight-party option.