Harry Seager, 34, unwrapped the chocolate bar he’d bought from a petrol station in the UK town of Thame last month only to find it was missing its signature ripple.
He posted an image of the smooth Mars bar to the popular Facebook group Dull Men’s Club and it attracted a flood of interest.
“Yesterday I had a completely smooth Mars Bar. I wonder if this is a cost cutting measure in this chocolate bar from now on,” Seager wrote.
One commenter described the unusual-looking bar as “unsettling” and another declared in was “hideous”.
Seager then emailed Mars Wrigley to ask why his chocolate bar looked so different.
He has since been given £2 ($3.90) in a voucher for his trouble.
“I think it’s amazing after everything that’s happened that I got the £2 voucher.”
A Mars Wrigley spokesperson told BBC that the smooth bar had “slipped” through the regular checks at its production line and had not been given the signature swirl.
“With over 2.5 million Mars Bars produced daily at our Slough factory it seems this has slipped through without its signature flourish,” the spokesperson said.
“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.”
Mars bars were first made by hand in Slough, Berkshire and they are still manufactured in the town today.