In the reported case of acute grade 3 anterior cruciate ligament injury, clinical examination and magnetic resonance imaging findings were in conflict, leading to confusion. Ultimately, dynamic ultrasound imaging proved to be the decisive test. The article describes the steps taken to reach the diagnosis. In addition, possible future diagnostic improvements are discussed. Dynamic ultrasound imaging, performed as part of the physical examination, is a valuable supplement to medical documentation. It provides appreciable diagnostic performance for the detection of anterior cruciate ligament insufficiency. Physical examination combined with magnetic resonance imaging, even though they represent the current diagnostic standard, have their limitations.