Most broken Sudokus come from one of six mistakes, and the biggest is guessing: putting in a digit that isn’t forced yet. The others are slips: checking a digit against only its row and column, trusting the colours too far, leaving a note out, letting old notes pile up, and seeing a pair that isn’t one. Each one is below, with how to spot it.
If you’re new, the rules and how to solve pages explain the ideas these mistakes go wrong on.
1. Guessing a digit before it’s forced
When a cell is down to two digits, it’s tempting to pick one and see what happens. Sometimes the grid breaks straight away and you know. But often it doesn’t, because every move after the guess follows honestly from it, wrong as it is.
Here’s an example. At the start of this Medium puzzle, row 7, column 5 can only be 4 or 7. Say you pick 7.
Nothing goes red, and the moves keep coming. Twenty-two digits later, each one placed correctly given the 7, row 6, column 8 sees all nine digits and has nothing left. By then 8 of those 22 digits are wrong too, and the broken cell is nowhere near the guess.
That isn’t a rare case. We took all 150 of our levels, put a wrong digit into a two-digit cell at the start, and played on with honest moves until something visibly broke:
| Level | Moves before it showed (median) | Still hidden after 20 moves |
|---|---|---|
| Easy | 0 (straight away) | 2% |
| Medium | 5 | 22% |
| Hard | 10 | 20% |
On Hard, 8 of the 50 guesses never showed at all: the moves simply ran out, with the wrong digit still in place and nothing visibly wrong. That looks exactly like being stuck on a hard puzzle.
How to avoid it: only place a digit when you can say why it’s the only option. If a cell is down to two, leave it and look for what decides it, in its row, column or box, or in the digit’s other places.
2. Checking only the row and column
To place a digit, it has to be missing from its row, its column and its box. Checking along the lines and forgetting the box is the classic slip when scanning fast, and it puts a repeat on the grid.
Checking both ways matters for notes too. In the how to solve walkthrough, row 1’s 4 looks as if it could go in column 3. Putting it there leaves the bottom-left box with nowhere for its own 4, so it’s wrong immediately, but only if you look at the box.
How to avoid it: say all three to yourself, “row, column, box”, for every digit you place, until it’s a habit.
3. Trusting the colours too far
On PeerPuzzles, a digit you enter turns red if it repeats in its row, column or box. That catches mistake 2 at once. But red only means “this breaks a rule right now”. A wrong digit that doesn’t repeat anything stays blue like every other digit, which is why the guess in mistake 1 went unnoticed for so long.
How to avoid it: treat the red as a safety net, not a checker. A grid with no red can still be wrong.
4. Leaving a note out
Notes are only useful when they’re complete. One missing note can make a pattern appear that isn’t there, and acting on it rubs out the right answer somewhere else.
In this row, column 2 can really be 3, 4 or 5. Leave the 5 out and it matches column 8’s notes, 3 and 4, exactly. That looks like a naked pair, so you’d cross 3 and 4 out of the rest of the row. But column 9’s answer is 3.
How to avoid it: when you write notes, go one cell at a time and check every digit from 1 to 9 against the row, column and box. Before acting on a pattern, recheck the notes in the cells it uses.
5. Letting old notes pile up
The opposite slip. When you place a digit, it has to come out of the notes of every cell in its row, column and box. A leftover note makes a digit look as if it has two places when it has one, so the moves are hidden. The game rubs these out for you when you place a digit, but on paper, or after an undo, it’s easy to miss one.
How to avoid it: after every digit you place, sweep its row, column and box for that digit in the notes.
6. Seeing a pair that isn’t one
A naked pair only works when both cells are in the same row, column or box. Two cells with matching notes, 2 and 8 say, in different rows, columns and boxes tell you nothing. A similar trap: two cells marked 2 and 8 in a row where a third cell also has 2 or 8 in its notes is still a pair. It’s the third cell that loses them. Getting that the wrong way round rubs out the pair’s digits instead.
How to avoid it: before using a pair, name the row, column or box it’s in, and cross out only in that one.
How to recover from a mistake
- A red digit repeats something in its row, column or box. Undo until it’s gone, then check the digit against all three again, the box as well as the lines.
- A cell with nothing left, or a digit with no place in a row, column or box, means a wrong digit or a wrong elimination somewhere earlier. It’s often a long way back. Undo to before your last guess, or to the last digit you were sure of.
- Stuck with no sign of a mistake? Check that your notes are complete first. Then work through when you get stuck.