During clean up following recent renovations at the Saxony city's St. Petri Cathedral archives, the head of the department discovered the large parchment, identified as a papal bull, or a document that contains a public decree by the pope. The church had no previous record of it being in the archives.
"The document lay there, carefully packed and unnoticed for decades in a cardboard box in a window nook," said archive director Birgit Mitzscherlich in a statement.
But the discovery of the Latin document has revealed another mystery for church authorities.
The elaborately penned letter to the archbishop of Besançon, France, in which Pope Innocent XI rules on the replacement of a member of an ecclesiastical chapter, is dated January 21, 1676. But this day predates Benedetto Odescalchi's election to the Holy See by nine months.
"We're still trying to figure that one out," Mitzscherlich told The Local. "But the letter notes that it is from his first year in the papacy."
Her only current explanation for the discrepancy is that it is a centuries-old error.
“The difference between marking the years is just one little character,” she told The Local. “It could be that the vital stroke was left out when the papal bull was written," she said, adding that the letter is definitely from the pope.
Mitzscherlich also isn't sure how the valuable document came to be among St. Petri's possessions.
"It's possible that the parchment was brought by German soldiers from France during wartime," she speculated.
Meanwhile Mitzscherlich has also found a seven-page letter from Otto Rudert, who held her same position as during World War II, which states that the document has been in St. Petri Cathedral's possession for the past 60 years. Rome apparently ordered it be returned to Besançon, near the Swiss border, but this was likely never done in the confusion of the post-war years.
Now the bull will be returned to Besançon, which Mitzscherlich says its rightful home.
"For the archdiocese in Besançon, this document is an important piece of its church's history," she said.
But for the next four weeks, people can see the 88 centimetre by 61 centimetre document — and the curious error it contains — on display in the St. Petri archive room.