In his first-century book Natural History, Roman naturalist and philosopher Pliny the Elder sang the praises of a sculpture located at the palace of Titus, Roman emperor from 79-81. He called the piece the Laocoön, writing that it was "a work to be preferred to all others, either in painting or sculpture." The sculpture, which Pliny believed was made from a single block of marble, was said to depict the legend of a Trojan priest named Laocoön, who was killed along with his two sons by sea serpents sent by the gods. Laocoön had been trying to warn his fellow Trojans about the suspicious horse lurking outside of their gates, which displeased Athena and Poseidon, who favored the horse-delivering Greeks.
Unfortunately, for many centuries, Pliny’s description was all that was left of the masterpiece. Then, in 1506, it was unearthed in Rome by a farmer digging up his vineyards. Michelangelo, among others, examined the statue and confirmed that it was the same one Pliny had described. Sadly, the fabled Laocoön (also called Laocoön and His Sons) hadn’t fully survived the test of time: It was missing the priest's right arm, among other pieces.
Respected artists of the day debated how to make the piece whole again. Michelangelo thought the missing arm had been bent back over the shoulders, trying to lift off the serpents. Others, including famed Renaissance painter and architect Raphael, believed the arm had been extended up and outward, as if pleading with the gods. (By the way, at least one art historian has since speculated that Michelangelo was entirely responsible for the sculpture, which would make the “unearthing” an elaborate prank.)
In 1510, the pope’s architect held a contest to see which artist could best complete the sculpture. The judge? Raphael. The Renaissance master awarded the work to sculptor Jacopo Sansovino, who (in line with Raphael's own beliefs) had created a version with an outstretched arm. But for reasons that are somewhat unclear, that version of the arm was never attached to the sculpture. An even straighter one, crafted by Michelangelo's former assistant, Giovanni Montorsoli, was added in 1532, and survived on the statue for centuries.
Fast-forward to 1905, when archaeologist Ludwig Pollak discovered the original missing arm in Rome, scattered in a stonemason's yard among a group of other marble body parts. He recognized that the style and age were similar to the Laocoön, and, suspecting that it was one of the sculpture's lost pieces, turned it over to the piece's current owner—the Vatican. Pollak was proved right when a drill hole was found in the arm that perfectly matched a drill hole in the shoulder of the sculpture. And the rediscovered arm was bent, as Michelangelo had originally suspected—not extended, as Raphael had thought. That meant the position of the Montorsoli arm, the one that had been attached to Laocoön's body for close to 400 years, had been incorrect.
The arm Pollak found was added to the sculpture in the late 1950s. But art enthusiasts who like the look of the outstretched arm more than the bent one don't need to worry. There are copies all over the world (like this one in Versailles) that still portray the old extended position—so you can still view it the way you (and Raphael) prefer.