Michelangelo only ever signed one of his sculptures, the exquisite Pieta in the Basilica di San Pietro. The patron saint of art historians, Giorgio Vasari (1511-74), explains why in his book Lives of the Most Excellent Painters, Sculptors, and Architects (first published in 1550 and enlarged in 1568).
“One day coming into the place where the Pieta had been installed, Michelangelo found a large number of strangers from Lombardy praising it highly. One of them asked another who had made it, and he replied, ‘Our Gobbo from Milan’. Michelangelo kept his counsel though it seemed rather strange to him that his painstaking work should be attributed to someone else. One night, bringing his chisels along, he locked himself in with a little light and carved his name there.”
The gobbo (ie. hunchback) was Cristofero Solari (c.1460-1527), a sculptor from Milan, who had achieved fame in Rome working for Pope Alexander VI (r. 1492-1503).
The inscription can be seen on the Virgin’s sash and reads: MICHELANGELUS BUONARROTUS FIORENTINUS FACIEBA (Michelangelo Buonarroti, Florentine, was making this). The word faciebat is carved as though disappearing behind the Virgin’s head dress and lacks the final ‘T’, an omission that has been taken as a subtle play on the term itself.
According to Pliny the Elder, it had been the practise of some ancient sculptors to inscribe the past continuous (faciebat/was making) rather the simple past (fecit/made) to imply that the artist had broken off unable to bring his work to the perfection of the image in his mind.