The King made a quip about his mishaps with fountain pens as he arrived in Scotland ahead of his summer break.
The comment came as Charles, 75, visited a giant peat bog recently given Unesco world heritage site status.
The monarch was given a tour of Forsinard Flows, Forsinard, Sutherland, on Wednesday, with the area estimated to store about 400 million tonnes of carbon dioxide, more than is stored by all UK woodlands combined.
As the King signed a book inside the visitor centre, he joked: "I'm hoping that my pen is going to work. It doesn't usually."
Watch the moment in the clip below…
The King famously aired his frustration over a leaky fountain pen, just days after his accession to the throne in September 2022 - a viral moment that was captured on camera.
Charles was heard exclaiming: "I can't bear this b***** thing" at Hillsborough Castle in Northern Ireland.
And after the realisation that he had signed it with the wrong date, he later stood up at the desk and said: "Oh God I hate this, oh look it's going everywhere, I can't bear this b***** thing, what they do every stinking time."
The King appeared to later poke fun at himself when he signed a visitor's book at a council meeting in Dunfermline, Scotland in October 2022.
As he handed his pen to his wife, he laughed and said: "These things are so temperamental."
For his latest visit to Scotland ahead of his traditional stay at Balmoral, Charles sported Highland dress including a Sgian Dubh – a dagger hidden in his sock – and had a cornflower in the lapel of his tweed jacket.
He unveiled a plaque commemorating the area last week becoming the seventh world heritage site in Scotland and the 35th in Britain.
Charles, a renowned nature lover who is patron of the Royal Society for the Protection of Birds (RSPB), met representatives of the charity, as well as from NatureScot – which led the bid for Unesco recognition – and Highland Council.
He also visited a nearby community festival, where he was given a saddle of venison to take home by caterers.
Admiring the firepit where venison burgers were being cooked and served with homemade condiments including blackcurrant ketchup, Charles joked: "I hope the venison burgers are up to standard."
A children's orchestra, Feis air an Oir – Gaelic for Festival on the Edge – also performed for the King, with many children playing the fiddle.
The King and Queen are expected to host their families at their Scottish residence, Balmoral, with the Castle closing to the public on 11 August.