As we’ve seen in a previous post, illuminations in incunabula can be seen as a remnant of the manuscript tradition that persisted in the transition to the printing era. In our copy of Ognibene Bonisoli’s De octo partibus orationis (Padua, 1474), bequeathed to King’s College by Jacob Bryant (1715-1804), there are no guide letters in the spaces left blank for the illuminator, so it was up to him to choose which letter to write. On the first page is Bonisoli’s dedication to his pupil, the condottiere Federico Gonzaga (1441-84), who later became the third marquess of Mantua.
Federico Gonzaga (right) in one of Andrea Mantegna’s frescos in the Camera degli Sposi in Mantua, painted between 1465 and 1474.
Instead of inserting an “E” to give “En humanissime pri[n]ceps” (Lo, most humane leader), the illuminator added an “I”: “Inhumanissime pri[n]ceps” (Most inhumane leader):
First leaf of Ognibene Bonisoli’s De octo partibus orationis (Padua: Bartholomaeus de Valdezoccho and Martinus de Septem Arboribus, 1474; Bryant.XV.3.6). The manuscript inscription at the top indicates that the book belonged to the church of Santa Maria Incoronata in Milan, which was completed in 1460.
Though this is most likely to have been an unintentional error on the illuminator’s part, whose Latin perhaps was not up to scratch, it is tempting to imagine that it may have been a parting shot from a disgruntled employee on his final day at work…
IJ
King's Treasures 
Interesting that you also have another incunable from Santa Maria Incoronata, a copy of Cicero printed in Milan in 1478, and also owned by Jacob Bryant. It’s described in Andriolo and Reynolds’s catalogue of Illuminated Incunabula in Cambridge, 2017, no 92.
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