A patient with crowns on #8 and #9 complains of mobility in the front teeth and muscle pain when opening the jaw. Overcontouring of which surface would most likely lead to this kind of failure?

The lingual incline of the maxillary anterior teeth provides anterior guidance during protrusive and excursive mandibular movements. Overcontouring this surface creates excessive/premature guidance contact, generating abnormal forces on the anterior teeth (contributing to mobility) and abnormal muscle activity/strain (contributing to jaw muscle pain) during function.