This paper examines the "stone of stumbling" theme in Finnegans Wake. The ambiguous nature of the stone —both foundation stone and stumbling block —is traced through the Old and New Testaments, the church fathers Barnabas, Pseudo-Dionysius, Chrysostom, and Origen, and the literary figures Swift and Beckett. For Joyce, what was a fatal flaw in the plan of the master builder became a methodology for discovery. Following Origen's dictum to "search the scriptures till you find a stumbling block," Joyce loaded his last work with offences and scandals. In this way he used stumbling, falling, and erring as "portals of discovery" in his never-ending linguistic adventure. He also passed on this technique to his great disciple Samuel Beckett —one of the master stumblers of the twentieth century.