Embrethiliel sends me the answer, found at Laudem Gloriae:
As it turns out, it is the work of Bosnian artist Braco Dimitrijevic (one of those modern conceptual artistes who deem themselves superior to that rabble of old-school representational artists), who, in 1971, thought it would be very clever to clandestinely lift one of the old paving stones and replace it with his own carefully inscribed tile. The point was to call into question society's assumption of the uniqueness or importance of the cathedral, challenging the observer to wonder why this place in particular was more important than, say, the loo in the café across the street. Any gormless nob with a third-grade education, of course, can answer that question, but probably not to the satisfaction of historical relativist Dimitrijevic. (One quotation gives the reader an idea of his views: "What we call History is nothing more than one subjectivity which is imposed on the whole world as objective opinion.")The cathedral enriches me (and many others throughout the years) by its existence; the loo only serves me. Unless, of course, it is a very beautiful loo.