BACKGROUND Cognitive errors are common in medical practice and can have serious consequences for patients related to misdiagnosis and delays in diagnosis and treatment. We report a case in which cognitive error substantially influenced a patient's diagnosis and treatment. CASE REPORT This report recounts the case of an adolescent girl with symptoms similar to those of her previous episodes of tetrahydrocannabinol-induced hyperemesis, despite a 21-kg weight loss over 6 months. She sought care at multiple facilities until finally being diagnosed with superior mesenteric artery syndrome. Treatment was conservative with intravenous hydration, nasogastric feeding, and gastric decompression until duodenal compression was relieved. The patient's condition had improved at 7-month follow-up, and she declined surgical consultation. CONCLUSIONS This case report seeks to raise awareness of the effects of cognitive errors in clinical practice. This tendency to prematurely attribute symptoms to a known diagnosis and thereby overlook alternative and potentially more accurate explanations can lead to delays in diagnoses and treatments. Awareness of cognitive error is especially important in the context of the increasing prevalence of legalization of tetrahydrocannabinol/marijuana in several states. Superior mesenteric artery syndrome, although rare, is linked to high morbidity and mortality when the diagnosis is delayed; thus, it is crucial to consider it in the differential diagnosis for a patient with weight loss and abdominal pain. By sharing this case, we hope clinicians and patients can become more aware of this rare consequence of tetrahydrocannabinol use to facilitate more comprehensive patient-centered investigations.