Coronoid process hyperplasia is a rare cause of mandibular hypomobility. It can result from temporalis muscle hyperactivity, trauma, and neoplasia, but often is idiopathic. Enlargement of the coronoid process leading to pseudojoint formation with the zygomatic arch is known as Jacob's disease. It results most commonly from an osteochondroma of the coronoid process. This is the first reported case of a non-neoplastic accessory mandibular condyle located at the coronoid process articulating with the zygoma.