There is an allure associated with a rare diagnosis. Diagnosis momentum refers to the “stickiness” of a diagnosis once it is attached to the patient,1 and is one example of a cognitive bias that can lead to diagnostic errors. This can lead to misattribution of symptoms to a patient’s existing (even incorrect) diagnosis if clinicians do not keep their initial approach adequately broad. We present a case in which a patient with long-term misdiagnosis of a rare problem was found to have a common condition that explained her symptoms.