miscount
To count something wrong and get the incorrect total.
To miscount means to count incorrectly, arriving at the wrong total. When you miscount the money in your piggy bank, you might think you have $47 when you actually have $52. When a teacher miscounts the students on a field trip, she might think everyone's accounted for when someone's actually still in the gift shop.
Miscounting happens easily when you're distracted, rushing, or dealing with large quantities. A cashier might miscount your change during a busy lunch rush. Election officials take extreme care not to miscount ballots because even small errors matter enormously in close races.
The word can apply to any counting error, whether you count too high or too low. A basketball scorekeeper who miscounts the points could change which team thinks it's winning. You might miscount the beats in a measure of music and come in at the wrong time.
Sometimes people miscount on purpose to deceive others, though usually it's an honest mistake. That's why important counting often involves double-checking: stores count their cash registers twice, scientists repeat their measurements, and careful bakers count their eggs before starting a recipe.