 The mark, if there's too many votes for each race, they reject that vote, not the entire ballot. It's just they reject that vote. Say for president, you get one choice and you vote for two. Your vote for president is rejected. You overvote it. Now you can undervote, leave them blank, or if you've got an office, say vote for three out of five and you only vote two out of five, your votes are still counted.