Why is my Sudoku number wrong when nothing repeats?

A Sudoku number can avoid every immediate repeat and still be wrong. Checking a cell’s row, column and box tells you which digits are currently allowed. It does not prove that every allowed digit can lead to a completed grid. In a Sudoku with one solution, only one of those candidates will work.

Some apps flag duplicate digits. Others compare an entry with the saved solution. That difference explains why an app can call a number wrong while you cannot see anything repeated. Here is a small deduction you can check yourself.

A legal-looking 8 that leaves nowhere to go

Rows count from the top and columns from the left. Look at r4c3, the outlined empty cell in row 4, column 3. Its candidates are 3 and 8. Neither digit already appears in its row, column or box.

Putting 8 in r4c3 would pass a duplicate check at that moment. But before entering it, examine the other outlined cell, r4c1.

1. Row 4 contains 2, 1, 6 and 4, leaving 3, 5, 7, 8 and 9.
2. Column 1 already contains 3, 7, 9 and 6. That removes 3, 7 and 9 from the list.
3. The middle-left box contains a 5 at r5c3. That removes 5.
4. Only 8 remains for r4c1. Place it there.

Now row 4 already has its 8, so r4c3 must be 3. If you put 8 in r4c3 first, r4c1 would have no candidate left. The contradiction is a stranded cell, rather than two equal digits already written on the board.

What the two kinds of checking actually check

In Puzzle Lantern, the optional Highlight conflicts setting marks repeated digits. Separately, an entered digit that differs from the solution counts as a mistake. So the absence of a highlighted conflict is not confirmation that an entry is correct.

How to avoid this kind of mistake

Keep a possible digit as a pencil mark until you can explain why it must go there. “There is no 8 nearby” is permission to consider 8. “Every other candidate is excluded” or “this is the only place for 8 in the box” is a deduction.

If a puzzle becomes impossible, return to the last placement you could justify. Rebuild the candidates from the large digits, because notes may have become incomplete or stale. A solver reporting an error does not necessarily mean the puzzle was generated badly, and an unchecked entry is not necessarily a safe one.

Next, try the worked guide to entering and updating pencil marks, or compare naked and hidden singles. The Puzzle Lantern Sudoku guide explains the 9×9 game, Daily grids and More puzzles.