King Charles admitted that he "likes being called darling" after an employee at Grimsby Town Football Club made a protocol blunder.
The King braved wet weather on Thursday during a visit to Grimsby and Cleethorpes, where crowds gathered with umbrellas and Union flags to welcome him.
At Blundell Park, home of Grimsby Town Football Club, the monarch received a personalised shirt bearing the number three and "HM King Charles" on the reverse.
The gift was presented by the club's mascot, the Mighty Mariner, with whom Charles exchanged a handshake.
King Charles admitted that he 'likes being called darling' after enduring a protocol blunder
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A light-hearted moment occurred when Francine Orr, the football club's ticket office manager, greeted the monarch in an unexpectedly casual fashion.
Despite being briefed beforehand on royal protocol, including proper forms of address and how to bow and curtsey, Ms Orr found herself slipping up when Charles approached her.
She said: "I've had it in my brain all morning, then he comes up to me, and he shakes my hand, and I said 'All right darling, oh no, I'm so sorry Your Majesty'."
The King took the gaffe in good humour, laughing and stepping back before telling her: "I like being called darling."
King Charles III reacts as he meets members of the 'Our Future' collective during a visit to Blundell Park, home of Grimsby Town Football Club
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Ms Orr described Charles as "so down to earth" and called him a "lovely, lovely man," expressing delight that he had chosen to visit the club.
Dozens of well-wishers stood in the downpour awaiting his arrival at the north-east Lincolnshire ground, some holding a banner featuring the King's face.
Residents in nearby homes watched from upstairs windows as the royal visit unfolded.
During his time at the stadium, Charles met representatives from the Our Future collective, a charity focused on citizen-led community transformation.
Simon Beeton, chief executive of local mental health provider Navigo, described the monarch as "clearly very well connected" following their conversation about community initiatives.
King Charles was all smiles meeting the mascot
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The King pledged to connect local organisers planning to plant 10,000 trees across Grimsby with his own head forester.
Mr Beeton said: "The guys who were looking at 10,000 new trees for Grimsby, for example, he's going to put them in contact with his head gardener. It's like, wow, he wasn't passively listening, he very actively listens."
He added: "The fact he's chosen to come to Grimsby to see the work we're doing here and got excited about it, he has made that commitment that he will send us the details of his head forester."
Crystal Ness, operational lead for Our Future in north-east Lincolnshire, told the Press Association that Charles was "impressed" by their efforts, which she said "validates" the work of local organisations.
Earlier in the day, Charles had visited the Horizon Youth Zone, a state-of-the-art facility where he chatted with schoolchildren, armed services cadets and scouts.
King Charles III talks with Our Future CEO and founder, Emily Bolton as he pays a visit to Blundell Park
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Young people showed him around the centre's climbing wall, sports hall and training kitchen, where he observed teenagers preparing pizza muffins and asked whether they minded missing lessons.
While touring the sports hall, the King joked: "I'm sorry we can't stop and play some badminton" and enquired about Grimsby Town's Carabao Cup triumph over Manchester United in August.
His final engagement was at The Care Hub, a community organisation offering housing and support services.
There he met privately with Liam Atkinson, 26, who came to the hub following a marriage breakdown and health difficulties.
Mr Atkinson said afterwards: "He's so down to earth. He's so kind. So considerate. He seemed to care, which is amazing."