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logical deduction – The off-by-one killer
I recently sent this killer sudoku to my dad and he managed to solve it fully before realising that one of his sums was off by one (think 7+7=13). All other rules and constraints were followed in his solution. How is this possible? What basic property of sudoku (regular sudoku rules, not killer-specific) is at play here? The cage in question is the 8-cage in middle left, which in his solution sums to 7 (shown in red).
