Cameron had been on his way to the Armed Forces Day celebrations in Plymouth, when he decided to stop for a coffee. He stunned locals by strolling into the Sandwich Box Plus cafe.
But Sheila Thomas, who was behind the counter, failed to recognise the Prime Minister and told him she was busy serving other customers.
After a ten-minute wait, Cameron's aides popped in to Warrens bakery next door to see if they could get served any quicker, and picked up the premier a jam doughnut and a cup of tea.
But when the Prime Minister stopped to enjoy his treat outside the shops, passers-by began to recognise him and he was soon back in the cafe chatting to members of the public, the Daily Mail reported.
Thomas was upset to see he had picked up his drink elsewhere and gave him another scolding.
"I didn't realise it was him at first. I was in the middle of serving a customer so I was running around trying to do something and didn't take too much notice to be honest," Sheila said.
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He asked if we served takeaway coffee and I said, "Yes but I'm in the middle of serving somebody" and he said, 'I'm so sorry'. It was only afterwards somebody told me it was the Prime Minister."
"He came back in afterwards and I told him off for not getting a coffee here. He shook hands and was very pleasant," she said.
It's not the first time the Prime Minister has been made to wait for a hot drink by a busy waitress as last summer while on holiday with his family in Tuscany, Cameron was forced to carry a tray of drinks to his table when the 27-year-old waitress told him she didn't have the time.