Britain's vented his frustration at a leaky pen on Tuesday during a signing ceremony in Northern Ireland, the second time in recent days he has shown visible irritation during such proceedings.
The King, visiting Northern Ireland as part of a tour to lead mourning for his mother Queen Elizabeth II, was greeted by cheering crowds and welcoming speeches, before delivering his own pledge to the region.
Shortly after, when signing a visitors' book in front of cameras at Hillsborough Castle near Belfast, the King reacted in frustration when a pen he was using leaked on his hand.
"Oh, God, I hate this [pen]!" King Charles said, standing up and handing the pen to his wife and Queen Consort Camilla.
"Oh look, it's going everywhere," the Queen Consort said as the King wiped his fingers.
"I can't bear this bloody thing ... every stinking time," King Charles said as he walked away.
One former aide told Reuters before Queen Elizabeth II's death that Charles could be fun, but was also short-tempered and demanding.
On Saturday when signing documents in London, an irritated King Charles gestured for aides to help when a pen holder on the table got in his way.
When completing the documents on Tuesday he also used the wrong date, before checking with an aide who told him it was 13 September, not 12 September.
