Can You Spot the Problem With This ‘Sudoku Challenge’ Magazine Cover?

Can You Spot the Problem With This ‘Sudoku Challenge’ Magazine Cover?
Imgur
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A Sudoku book cover has people scratching their heads.

The cover for “Sudoku Challenger: Big 360: includes a basic mistake.

Did you see it?

The hand has just written a 5 in one of the squares--but there’s already a 5 in the block.

(<a href="http://imgur.com/gallery/F7x8lJ5" target="_blank">Imgur</a>)