There is a particular feeling when a sudoku stops making sense — two cells in a row both need a 4, or a box has no home for a 7. Almost always the cause is not the current position but something entered much earlier. These are the seven usual culprits.
1. Stale candidates
You place a 6 and forget to remove 6 from the candidates in its row, column and box. Every deduction that touches those cells is now unreliable, and the error surfaces ten moves later somewhere unrelated.
On paper the defence is a rigid habit: place a digit, then immediately sweep its three regions before doing anything else. On this board the sweep happens automatically, which removes the entire category of error.
2. Partial pencil marks
Marking candidates in some cells and not others leads to false conclusions, because an unmarked cell looks like it has no constraints. If you are going to use candidates, do the whole grid; if not, use none.
3. Guessing and not tracking it
If you pick one of two possibilities and carry on, you need to remember where the guess was so you can unwind it. People rarely do. Twenty placements later the grid contradicts itself and there is no way back except starting over.
Since every puzzle here is guaranteed to have a single logical solution, guessing is never necessary. If you are tempted, the answer is a missed hidden single far more often than a genuinely ambiguous position.
The undo button is not a time machine for logic
Undo steps back through your entries, but if you have been guessing for twenty moves you will not remember which one was the guess. Avoid the situation rather than relying on the escape.
4. Assuming symmetry or patterns
Sudoku grids often look symmetrical because generators remove clues in mirrored pairs — this one does. That symmetry applies to the clue layout, never to the solution. There is no rule that the answer forms a pattern, and expecting one leads to placements that feel right and are not.
5. Working cell by cell
Staring at an empty cell and asking what belongs there means checking nine digits against three regions. Picking a digit and sweeping the grid means checking one thing at a time. The second is several times faster and produces fewer errors, because you are holding less in your head.
6. Trusting a deduction you cannot articulate
If you cannot say why a digit goes somewhere in one sentence, do not place it. 'It feels like a 4' and 'it must be a 4 because the other cells in the box all see a 4' are very different states, and only one of them is reliable.
The hint button here exists partly for this: it names the technique, so you can compare your reasoning with the actual justification.
7. Pressing on while tired
Accuracy falls off sharply in the last third of a long grid, precisely when the candidate lists are shortest and the deductions are most fragile. Walking away and returning is usually faster than pushing through, and the puzzle is saved automatically here so nothing is lost.
Recovering from a broken grid
- Turn on mistake warnings. They mark every entry that contradicts the solution, which localises the damage immediately.
- Clear the marked cells and everything you derived from them.
- Rebuild the candidates from scratch rather than trusting what is there.
- Re-scan all nine digits before attempting anything advanced.
It is worth resisting the urge to restart entirely. Most broken grids have one bad cell, and finding it teaches you more than a clean run would.