A man unwrapped a Mars bar to find it completely smooth - without the signature ripple.
Harry Seager, 34, bought the chocolate snack from a services in Thame, earlier this month.
The broadcast worker, from Aylesbury, said he initially thought the lack of ripple was a new "cost-cutting measure".
But Mars has confirmed it was a one-off mistake - and the classic design is here to stay.
Harry has since posted a photo of the bar on the Dull Men’s Club group on Facebook - where it has divided opinion.
He said of the initial find: “Initially I thought maybe they had as a cost cutting measure taken the ripples out.
"Then I showed my friend, and he said that he had eaten one the day before which had a ripple in it.
“We then worked out that it was more than likely a manufacturing defect. It still tasted exactly the same.”
Mr Seager then took to the Dull Men’s Club group on Facebook to share his amusing find and was shocked to see how popular the post became.
The photo has now received over 15,000 reactions and 3,000 comments from users dubbing it "unsettling" and "hideous".
Mr Seager added: “I took the picture and then I had pretty much completely forgotten about it until the next morning.
“I was on the Tube to work, and I thought the Dull Men’s group would really appreciate the photo. So I posted it and thought nothing of it.
“I checked my phone, and it had gone absolutely crazy. I was surprised by how many likes and shares it had gotten."
A Mars Wrigley UK spokesperson said: “Our chocolate scientists have been perfecting the Mars Bar and its iconic swirl on top for over 90 years.
"With over 2.5 million Mars Bars produced daily at our Slough factory it seems this has slipped through without its signature flourish.
“While we can't reveal all the secrets behind our product line, we can promise fans that these ‘smooth’ Mars Bars are a rare find and the classic swirl isn’t going away.”
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