Puzzle fans are aghast after an eagle-eyed customer spotted a mistake on the cover of a Sudoku book.
The mistake was posted on Imgur, and has attracted a flurry of comments by amazed Sudoku enthusiasts.
The image includes a hand filling in the squares, although the person seems to have made a miscalculation.
On the cover, the person is writing a number 5 in the 3x3 box, however, there is already the same number in that block.
Fans of the popular Japanese game know it requires the player to fill in all the squares in the grid so that each row, column and each of the 3x3 squares contains all the digits from 1 to 9 without repeats or omissions.
The image has been viewed almost two million times since it was uploaded last week.
One viewer also commented on the unusual Japanese character used on the cover of the puzzle book.
"Also the random kanji they picked has nothing at all to do with sudoku. It means hue or color."
Sudoku players say you don't need to be a maths whizz to complete the puzzles, but a keen grasp of logic and plenty pf patience goes a long way.