The Most Common Sudoku Mistakes and How to Fix Them
By Michel ·
Sudoku a Day Blog
You made a mistake. The entire puzzle falls apart. Again. What went wrong?
Here are the most common mistakes and how to avoid them.
1. Not Checking All Three Constraints
A candidate must fit its row, column and 3×3 box. Checking only one unit can miss an immediate conflict. Even a digit that creates no repeated number may be wrong, so also explain why the other candidates are impossible.
Always verify all three before placing a number.
2. Guessing Instead of Reasoning
An unproved placement can make later deductions unreliable. If you notice a contradiction, return to the last state you can justify and recheck the affected candidates. You do not necessarily need to restart the whole puzzle.
When stuck, check your candidate notes and look for a deduction you can explain. A technique you know may not reveal the next move; consult a worked lesson or try a less demanding puzzle rather than treating an unproved placement as certain.
3. Not Using Pencil Marks
Pencil marks are optional, but they can help you track candidates when mental bookkeeping becomes difficult. Use them to make your reasoning easier to check.
Keep notes consistent with the current grid. Remove a candidate only when a rule or deduction excludes it, and revisit notes affected by a corrected placement.
4. Making Assumptions About Numbers
"The 5 must go there because..." Then it turns out you miscounted. Always double-check before placing.
Before committing a digit, check the reason for the placement and the cells involved. Taking time to verify a step can save retracing uncertain moves later.
5. Not Taking Breaks
If you find yourself repeatedly checking the same cells without progress, consider taking a break. There is no fixed solving duration at which everyone loses concentration.
Pause when you want to and return when you feel ready. If app interruptions bother you, compare Sudoku apps and check their current advertising and notification settings.
6. Playing at the Wrong Difficulty
If a puzzle feels frustrating, try a different difficulty or practise one technique separately. Difficulty labels are guides, not an assessment of your ability.
Move between levels at your own pace. You can learn from a harder example while still enjoying simpler puzzles.
A practical checking routine
Check the row, column and box; justify each placement; and keep any notes you use up to date. If something goes wrong, retrace the uncertain step before continuing.
Try today's puzzle and practise explaining each placement before entering it.