<p>The American College of Cardiology’s Adult Congenital and Pediatric Cardiology Quality Network has developed a metric for assessing comprehensive transthoracic echocardiographic (TTE) exam. This study evaluates the relationship between the study comprehensiveness score (CS) and diagnostic errors in patients undergoing congenital heart surgery. The echocardiography lab quality database at a single-center identified preoperative TTEs with diagnostic errors when compared with transesophageal echocardiograms or direct surgical inspection from 1/1/2018 to 8/1/2023. TTEs without diagnostic errors served as controls (similar age, gender, and surgical risk score). TTEs were assigned a CS by a single blinded reviewer. Diagnostic errors were characterized by types of errors, primary contributors to errors (procedural/conditional, cognitive, technical, patient-and-disease-related factors), severity, and preventability. Of the 2448 TTEs, 159 (6.5%) had diagnostic errors. There was no significant difference in CS between those with diagnostic errors compared to controls (95.4% vs. 93.1%, <i>p</i> = 0.06). CS was significantly associated with contributors to errors (<i>p</i> = 0.004). Cognitive factors were the most frequent contributors to errors (51.6%), followed by technical factors (33.4%), procedural/conditional factors (7.5%), and patient-and-disease-related factors (7.5%). CS &gt; 90% was associated with cognitive errors (<i>p</i> = 0.042), which contributed the most to errors. CS ≤ 90% was associated with procedural/conditional errors (<i>p</i> = 0.001). Application of the CS did not help discriminate studies with diagnostic errors. Cognitive errors accounted for half of the errors and had a high CS. While the CS may prevent some procedural errors, these constitute a minority. Future quality interventions should target cognitive errors.</p>