The patient was a 25-year-old man who was serving in the military. He was referred to a physical therapist for a progressively worsening painful snapping sensation in the posteromedial left knee region. Prior magnetic resonance imaging for the left knee was interpreted as normal, except for a mild fluid signal about the left pes anserine bursa. The patient was diagnosed with snapping pes anserine syndrome. However, despite 6 weeks of physical therapist intervention, the patient did not improve. A dynamic real-time ultrasound examination was then ordered, which demonstrated that the pes anserine tendons changed position as the knee moved from flexion to extension, sliding across a heterogeneous rounded mass in the posteromedial knee.