Cutwell had said. In the Disc model, Ankh-Morpork was a carbuncle. Alberto Malich, Founder of This University. Amongst other things he taught at the University of Paris, was Bishop of Regensburg, and at the age of 84 he again undertook the long journey from Cologne to Paris to defend the scientific work of his greatest student, Thomas Aquinas, against attacks and misunderstandings. I don't even remember walking under a mirror.
Superstition says that both walking under a ladder and breaking a mirror give bad luck.
Therefore, by the sort of skewed logic Terry continually gives to his characters, walking under a mirror must be really bad news. Ilium is the Latin name for Troy.