Common Sudoku mistakes

The habits: rescan after every placement, keep notes in step with the board, verify the cells and counts a pattern needs before acting on it, and use the game’s mistake feedback while you are learning.

Why do mistakes hide?

Sudoku punishes errors late. A wrong digit rarely breaks a rule at once. It sits in the grid, narrows the candidates of its neighbors, and produces more placements that look valid because they were derived from it. The contradiction surfaces many moves later, when some cell has no digit left or a digit has no cell. By then it is hard to tell which move was the bad one. For that reason the best defense is to avoid the error at its source, and the groups below are ordered roughly from the most common to the most subtle.

Mistake 1: guessing instead of deducing

Every puzzle on this site has exactly one solution and can be solved by deduction, so a guess is never required. A guess can be right by luck and wrong by chance, and a wrong one corrupts everything that follows. The better response to a stall is to go back over the cheap techniques, which the strategy guide lists in order, and then look at the next technique tier. If you must test a hypothesis, treat it as an experiment: note the cell, work out its consequences, and undo the lot if a contradiction appears. Never leave a hypothetical digit sitting in the grid as if it were certain.

Mistake 2: stopping when the singles run out

Many beginners reach a point where no cell has just one candidate and decide the puzzle is stuck. The next step is not to guess but to look at the board from the other side: instead of asking what each cell can hold, ask where each digit can go. That is the hidden single, and it often unlocks the grid. After that comes the line-and-box interactions and the pair techniques in the techniques hub. A puzzle is stuck only when every technique you know has been tried.

Mistake 3: not rescanning after a placement

A new digit changes up to twenty cells, those sharing its row, column or box. Players who scan the grid from the top left every time spend their effort on cells that did not change. Start from the neighbors of the digit you just placed. The guided first puzzle shows a case where one placement turns a cell with two candidates into a single.

Mistake 4: misreading the board

• Forgetting the box. Rows and columns are easy to run an eye along. The box is the group that gets overlooked, so check it every time you count what a cell can hold.
• Counting a filled cell as open. When checking where a digit can go in a row, a cell that already has a value is not a candidate place. This is easy to do by accident when scanning quickly.
• Missing a blocking digit in the crossing line. A cell looks open for a digit until you notice that the same digit sits elsewhere in its column or box. Check both the row and the column of every open cell before declaring a single.
• Swapping row and column. This site writes cells as r3c7, meaning row 3, column 7, with rows counted from the top and columns from the left. Reading it the other way round leads to deductions about the wrong cell.
• Confusing look-alike situations. A digit that fits in exactly two cells of a group is not a hidden single, and two cells that share two marks are not a naked pair unless the marks match exactly.

Mistake 5: wrong or stale pencil marks

Notes are an aid only when they are accurate. Most of the notes-related errors are predictable.

• Not updating marks after a placement. A leftover mark for a digit that is now in the same row, column or box is false, and it can hide a single or invent a pair. The game can remove such marks automatically when you place a digit, using the setting called “Remove notes automatically when placing a digit”.
• Pressing Auto-notes after eliminating by reasoning. Auto-notes recomputes every empty cell from the placed digits alone, so it restores candidates you had removed using pairs or lines. Use it once, at the start of marking.
• Leaving notes mode on. With it on, tapping a digit adds a small mark instead of a value. If a digit does not appear as you expect, check the Notes button.
• Letting a wrong digit damage the notes. If you place a wrong value, the automatic removal strips the digit from neighbors’ notes. Undo restores both the value and the notes.

The pencil marks guide covers these habits in detail.

Mistake 6: applying a technique to the wrong cells

Reading across the twelve technique lessons, the slips fall into a handful of kinds, and the same ones recur from simple patterns to advanced ones.

• Miscounting. A pattern needs an exact number of cells or digits. A hidden pair needs two digits confined to two cells. An X-Wing needs the digit in exactly two cells in each of two rows. A naked triple needs the three cells together to hold only three digits. A third candidate in the wrong place, or a fourth digit, breaks the pattern.
• Erasing from the wrong cells. A pattern removes candidates from the cells around it and keeps its own. In an X-Wing the four corners keep the digit. In a naked pair the two cells keep their own two marks. In a pointing pair the cells of the box keep the digit and the line outside the box loses it.
• Using the wrong group. The elimination applies only to cells that see all the cells of the pattern, through the same house. A cell that sees only one of them is unaffected.
• Checking one orientation. Techniques such as X-Wing and box/line reduction work on rows and on columns, and a search that covers only one of them misses half the patterns.
• Using a pattern from stale notes. A pattern built from marks that no longer match the board is a false pattern.

A useful routine is to confirm two things before acting on any pattern: that each cell you rely on really carries the candidates you think it does, and which cells will lose a candidate. Say them aloud or write them down if it helps.

Mistake 7: trusting a shortcut that needs a promise

The unique rectangle removes a candidate because keeping it would allow two solutions. That reasoning works only if the puzzle has exactly one solution. Puzzles on this site are checked for uniqueness before release, as described in how puzzles are generated and verified unique. A puzzle from a magazine, an app or a hand-made grid may not be, and then the shortcut can lead you wrong. Prefer a technique that needs no assumption whenever one is available.

Which mistakes are specific to the variants?

Each variant adds a rule that is easy to forget in the middle of a solve. The rules themselves are on the rules page, and the slips below are the typical ways of breaking them.

• Diagonal Sudoku: checking rows, columns and boxes but forgetting that both main diagonals must also hold every digit once. The diagonal board has more constraints on the cells that lie on them.
• Anti-knight Sudoku: forgetting that two equal digits cannot be a chess knight’s move apart. This rule removes candidates in a pattern that is easy to miss by eye, so check it for any cell you are about to fill on the anti-knight board.
• Killer Sudoku: forgetting that a cage must add up to its target sum and that no digit may repeat inside it. Both conditions limit what a cage can hold, on the killer board.
• 16×16 Sudoku: the symbols run from 1 to 9 and then A to G, so a letter is a legitimate digit and a common source of transcription slips on the 16×16 board.
• Irregular Sudoku: assuming the regions are 3×3 boxes. On the irregular board the region of each cell has to be read from the outlines.

Habits that prevent most errors

1. Say what you are about to do before you do it: “the 4 in row 3 can only go in column 7”. A claim you can state is easier to check.
2. After every run of five or six placements, look over the board for duplicates in any row, column or box.
3. Use mistake feedback while you are learning, and move to Off only when your errors are rare.
4. Take a break when you start placing digits quickly without checking them. That is the point when errors are most likely.

How does the game help you catch errors?

The mistake feedback setting has three options.

• Instant. Conflicts and wrong digits are marked as you play. This is the easiest setting for learning, because an error shows up as soon as you make it.
• Off. Nothing is marked while you play. You can still run a check on demand, which flags every wrong digit until the board next changes, and the check is never counted as a mistake.
• Strict. Three mistakes ends the game.

A conflict and a wrong digit are different things. A conflict means the digit repeats within a row, column, box or another region of the board. A wrong digit means it differs from the puzzle’s one solution, which may not create a visible conflict until later. That is why a wrong digit can sit unnoticed with feedback off. The heads-up display counts wrong digits as mistakes, and the controls page lists where to change the setting.

What if you have already gone wrong?

1. Stop placing digits. Each further placement built on a wrong digit makes the search harder.
2. Use undo to step back through your moves. It restores values and notes together and can go back a long way.
3. Check each digit against its row, column and box, starting with the most recent. A digit that conflicts, or a cell with no candidates, marks where the damage is.
4. When you find the first wrong move, undo to just before it and take a different route.

If you cannot find the error, restart the puzzle. It costs less than another hour of work on a corrupted board. When you are ready to play, pick a level from the home page, and keep the guides hub in mind for the other tutorials.