STORY: Charles, alongside the Queen Consort Camilla, was attending a reception at the castle, where he met with members of Northern Ireland's assembly.
During a book signing ceremony towards the end of visit, Charles complained about the pen he was using, with the Queen Consort suggesting it was leaking.
The pen was swiftly removed by a royal aide before Camilla sat down to sign the book herself.
It is the second time King Charles III has made headlines over a pen. On Saturday (September 10) he was officially proclaimed as Britain's new monarch at a historic ceremony in St James's Palace.
During the signing ceremony, Charles was seen to prompt his aide to remove a tray of pens off his desk before continuing to put pen to paper.