Nobody, but nobody, was to see his ceiling until he had finished it, but he suspected that his assistants had been bribed to let someone in.
What he didn't realise was that the interloper was Pope Julius II, the very man who had commissioned the painting.
As the prying pontiff raised his eyes upwards, hoping for an illicit preview of Michelangelo's sumptuous religious imagery, he was greeted instead by the sight of heavy planks tumbling from the scaffolding and crashing down on the floor all around him.
The Pope narrowly escaped being hit and was driven out of the chapel by Michelangelo's fury, but he could hardly blame the artist for the attack since he had been guilty of many violent outbursts himself during the four fractious years it took to paint the ceiling.
Towering achievement: The Sistine Chapel - from the creation to Noah in 175 individual paintings covering 12,000 square feet
This year marks the 500th anniversary of Michelangelo's first brush-strokes on this most turbulent of projects, remarkable not just for its great beauty and technical accomplishments but for the fact it was finished at all. One of the world's most popular tourist attractions, the chapel draws millions of visitors to the Vatican every year, but few who stare up in awe at its ceiling are aware of the unholy conflict between painter and patron which lay behind its creation.
This enmity was perhaps inevitable, given the personalities of the two men involved.
Julius II, or the "Warrior Pope" as he was known, was a demanding and imperious man, given to wearing a suit of silver armour as he marched his armies up and down the Italian peninsula.
As for Michelangelo Buonarroti, he was a genius whose fiercely independent and often perverse personality had been shaped by a traumatic childhood. Like many children from families of education and social pretension, Michelangelo had been handed to a wet nurse soon after he was born in 1475.
He did not return to the family home in Florence until he was two, and when he was six his mother died.
Having lost his surrogate and natural mother in quick succession, he soon encountered difficulties in his relationship with his father, Lodovico.
Once prosperous money-lenders, the Buonarroti family had fallen on hard times, yet Lodovico was a snob who still looked down on people who worked with their hands, including artists, who were then regarded as little more than glorified craftsmen.
When his son's obvious talents for drawing and sculpture led him to seek out the company of such people, Lodovico feared he would bring disgrace to the family. He and Michelangelo's four brothers frequently beat the boy because of his interest in art.
Fortunately, his father's snobbery also worked in the young Michelangelo's favour. At 15, the boy's talents as a sculptor were spotted by Lorenzo de' Medici, head of Florence's leading family, who invited Michelangelo to live with him and his family and study at a new art academy he had founded in the grounds of his estate.
Summoned by Lorenzo to discuss the matter, Lodovico could hardly resist an invitation to meet his social superior. He accepted a job in the city's customs office in return for giving up his son.