The pen mishap only added to the tense atmosphere of the signing, with an obviously-distressed Charles also getting confused with the date.
After finding out that it was September 13, the King revealed that he had signed the documents with the wrong date - September 12.
It’s the second time in recent days that the new king has become visibly frustrated.
Earlier this week he again got flustered over pens, gesturing to aides to remove a pen holder which was in his way in a video which later went viral online.
The momentous week has also seen King Charles make his first official speech as sovereign - in which he paid tribute to his late mother, Queen Elizabeth.
Along with awarding his son Prince William and daughter-in-law Princess Catherine his and Camilla's old Prince and Princess of Wales titles, the latter of which has not been used since Princess Diana's lifetime, Charles also extended an olive branch to his second son Prince Harry and his wife Meghan Markle.
"I want also to express my love for Harry and Meghan as they continue to build their lives overseas," the King said.
It was the only sentence in the speech that paid mention to the Duke and Duchess of Sussex.
While Harry and Meghan have stepped away from their senior royal duties, the Queen's death does prompt some royal changes when it comes to their children, Archie and Lilibet.
The three- and one-year-old now adopt Prince and Princess titles respectively.