The library of Raphael de Mercatellis (1437-1508), natural son of Philip the Good of Burgundy and abbot of St Bavon in Ghent, contains two manuscripts that are similar in both design and content. The manuscripts are preserved as Ghent, UL, MSS 11 and 17. Both were ordered by De Mercatellis around 1500 and contain as main text the Monotessaron, a well-known Gospel Harmony by Johannes Gerson. The text is in both cases supplemented with an extensive set of fragments and glosses, and more than 130 full-page miniatures. Codicological analysis of the manuscripts reveals what went wrong in the planning and copying process of MS 17, because of which the production of MS 11 became inevitable.