Vaccination-related errors reported to the National Vaccine Adverse Event Reporting System grew from 10 in the year 2000 to 4324 in 2013, potentially due to the introduction of new vaccines, increasingly complex vaccination schedules, and changes in reporting practices. The most common errors were dispensing vaccines at an inappropriate schedule or administering expired or incorrectly stored vaccines. One-fourth of reported errors caused an adverse health event, with 8% of these resulting in serious harm.