The Beauvais Sacramentary (Los Angeles, J. Paul Getty Museum, MS Ludwig V 1) comprises the surviving ten leaves of a deluxe liturgical manuscript almost certainly created at the behest of Roger, bishop of Beauvais (998–1016). According to tradition, its parent manuscript was one of three liturgical codices left to the cathedral church of Saint Peter by Roger. A half century or more after the manuscript was originally copied, a single scribe added new text to the canon of the mass on top of an erasure, in a margin, and on an inserted slip of parchment. This paper explores the character of the emended text and the motivation for the interpolations. It is proposed that the intervention was most probably made in preparation for the dedication of the Augustinian church of St. Quentin-de-Beauvais by bishop Guy of Beauvais on Sunday, October 4, 1069.