Clinical decision support (CDS) includes electronic alerts that can prevent errors. Excessive or erroneous alerts may lead to alert fatigue or other unintended consequences. Researchers used a blend of qualitative methods such as interviews and quantitative data like alert rates to develop a taxonomy of CDS alert errors. The taxonomy includes the origin of the error, which most commonly occurred with introduction of a new decision support rule, and describes the underlying reason for the error, such as problems with new terms, conceptualization, and building the rule as intended. Errors could cause an alert to fail to appear for a relevant situation or could cause an irrelevant or erroneous alert to appear. Most errors came to light through reports from users. The authors recommend classifying CDS alert errors using this taxonomy so that safety efforts will be consistent and actionable.