How to Find a Mistake in Sudoku (Without Starting Over)

To find a mistake in sudoku, look for one of three signs: the same digit twice in a row, column, or box; a cell where no digit fits; or a digit that has no legal place left in some row, column, or box. Any of them proves that a digit you placed earlier is wrong. The wrong digit is almost always one of the digits that rules out the empty spot, so start there, check how you got each of them, and undo from the most recent. You rarely need to start the whole puzzle over.

Sign 1: a duplicate

The obvious one. Scan each row, column, and box for a repeated digit. It takes about a minute on paper if you go box by box and count 1 to 9. A duplicate tells you exactly where to look: one of the two copies is wrong, and usually it is the one you placed more recently.

The trouble is that most mistakes never produce a duplicate. A wrong digit placed in a legal spot looks fine. It only shows itself later, when it squeezes some other cell or digit out of existence. That is what the next two signs catch.

Sign 2: a cell with nowhere to go

If the digits in a cell's row, column, and box already cover all nine digits between them, that cell has no candidates left. The puzzle cannot be finished from here, so something on the board is wrong.

In pencil-mark terms, this is a cell whose last candidate you just erased. If you keep full pencil marks, you see it the moment it happens: an empty cell with nothing written in it. That is one of the best arguments for keeping full pencil marks on hard puzzles.

Sign 3: a digit with no home

This one is sneakier. Every row, column, and box needs each digit once. If some group has no empty cell left where a missing digit could go, there is a mistake, even if every single cell still has candidates.

Notice that the wrong digit is not in row 3 at all. It sits in the column and the box that cross the row. That is the general rule for tracing a mistake: the culprit is among the digits that see the problem cells.

How to trace the wrong digit

1. List the suspects. For an empty cell, the suspects are the digits in its row, column, and box. For a digit with no home, they are the copies of that digit that block each empty cell of the group. Usually that is only two to four digits.
2. Ignore the clues. The printed clues are never wrong. Only digits you wrote in can be.
3. Check the newest first. For each suspect you placed, ask how you got it. A hidden single you can still re-derive is fine. A digit you cannot justify from the current board is your prime suspect.
4. Undo in reverse order. Erase the suspect and every digit you placed after it, then continue from there. Losing ten moves is much better than losing the puzzle.
5. Audit the pencil marks nearby. A candidate erased by mistake does the same damage as a wrong digit: it can make a wrong cell look like a single. Rebuild the marks in the rows and boxes around the problem from scratch.

If you use a pencil, a simple habit makes this much easier: write each new digit a little smaller, or tick the ones you have double-checked. Some solvers put a dot in the corner of a cell whenever they place a digit on an assumption, so they know exactly what to erase.

Where mistakes usually come from

• A stale pencil mark. You placed a digit but did not erase it from the other cells in its row, column, and box. Later a cell looks like it has one candidate when it really has none of the ones written.
• Scanning the wrong box. Reading a 6 from the box next door when checking a hidden single.
• The right digit in the wrong cell. As in the first example above: you knew the row needed a 5 and put it in the first open cell that allowed one.
• A half-applied technique. For a naked pair, eliminating the pair's digits from the pair cells themselves, or from a group they do not both belong to.

Is the puzzle broken instead?

Almost never, if it comes from a newspaper, a puzzle book, or a good app. Before blaming the puzzle, check the clues against the source. A single misprinted or miscopied clue is far more common than a broken puzzle. The one real failure mode is a puzzle with two solutions, which shows up as four cells in a rectangle that can hold the same two digits either way. See can a sudoku have more than one solution. And if the board is correct but you simply cannot find the next move, that is being stuck rather than a mistake: our stuck checklist covers that.

Mistakes in Sudoku Royale

In the app, a wrong digit never gets onto the board. Each entry is checked against the solution the moment you place it, and a wrong one is rejected: the cell shakes and stays empty. So the whole problem on this page, a hidden error that surfaces twenty moves later, cannot happen. In Classic mode mistakes are unlimited and only counted on the results screen. In the Daily Challenge you get three lives, and each mistake adds 30 seconds to your time. Placing a digit also clears it from the pencil marks in its row, column, and box automatically, which removes the most common source of errors.