Sudoku Mistakes

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Published May 1, 2026 · Updated October 20, 2026

In our games a wrong digit cannot hide: it turns red the moment it goes in, whether or not anything clashes with it. On paper, or in an app that only flags a digit when it repeats one in its row, column or box, the same digit can look right for a long time, and we measured how long. The mistakes that do slip past our games live in the pencil marks, where nothing checks them.

What happens when you make a mistake in our sudoku games?

Every game on this site with a Mistakes count, from classic sudoku to killer sudoku, counts the same way:

• Each digit you place is compared with the puzzle’s answer. A wrong one is drawn in red straight away and adds one to the count, even if no digit in its row, column or box is the same.
• The count only goes up. Undo or Erase takes the digit away but not the mistake, and putting the same wrong digit back counts it again.
• There is no mistake limit. In our test the count reached twelve and the game carried on; it resets when you start a new puzzle.
• Hint is not counted. On most of the games it picks a square that is empty or wrong and puts its answer in, without saying why; Kropki’s Hint shows a step of logic instead.

On our classic pages, a right digit can be drawn red too: place a 9 wrongly, then the real 9 in the same row, and both are red, because they clash. Only the first is counted. Killer sudoku colours only the wrong one.

Checking against an answer works only because every board has exactly one. Our classic generator removes a digit only while its own logic can still finish the board, and a board that logic finishes has exactly one answer (how we make and test puzzles).

How can a wrong digit look right?

A wrong digit breaks no rule when it goes in; it only breaks the puzzle later. Most often it starts with a pencil mark that was never written. Here R9C8 shows a single 5 because its 6 was missed:

Nothing about the 5 looked wrong when it went in, and nothing about the next 20 digits did either. The board broke in two places, and neither is where the mistake is. In our game the 5 would have been red at once; on a printed puzzle it is found only by working back.

That board is not unusual. We took 200 boards from each of our levels and, at the first move, tried every wrong candidate in turn, then took singles in reading order until the board showed it was broken: a square with no candidate left, a house left with nowhere for some digit, or two squares of one house down to the same digit.

Every wrong candidate at the first move of 200 boards per level (11,474 to 34,386 runs per level), singles taken in reading order after it, October 2026. “Digits placed” is the median; “away” means the break was in no row, column or box of the wrong digit. On Hard and Expert, 0.6% and 3.7% of runs stalled with nothing broken.

The harder the level, the longer a wrong digit survives, and the more company it gets: on Expert, the digits placed before the break include close to three more wrong ones on average. Most of a square’s candidates are wrong, too, so a digit chosen because it “fits” is usually a wrong one.

Can a wrong elimination ruin a sudoku?

Yes, and on our games it is the one mistake nothing flags, since only digits are checked. The classic case is a pointing pair that isn’t one: two places for a digit in a box line up, and the third place goes unseen.

The misread pair takes the 2 out of R5C4 and leaves a 4 that looks like a naked single. Place it and our game turns it red; on paper it is just the next digit. The guard is a habit: before striking anything with a pointing pair, count every place the digit has in the box, not the ones that line up. The real pair at this moment, box 4’s 4s, passes that count, and the same check applies to every locked candidate and pair.

Do you need X-Wings on Hard and Expert sudoku?

Not on ours, and assuming you do is a mistake in itself: time spent hunting a fish is time not spent on the move that is there. Our generator deals a board only if a fixed set of steps finishes it. Across 200 boards per level, solved with the simplest move at every step:

• Easy and Medium: singles finished all 400.
• Hard: 23 of 200 also needed a locked candidate (a pointing pair or a box/line reduction). Nothing else.
• Expert: 68 of 200 needed more than singles, and 17 of those a naked pair. Nothing else.
• X-Wing: needed on none of the 800. One was on the board at 28 of the 59 moments when singles ran out on Hard, and 138 of the 277 on Expert, but a simpler move was always there too.

The X-Wing starts to matter on our Evil puzzles: our X-Wing guide counts 644 of the 5,000 that need one. So on Hard and Expert, when singles run out, look for a pointing pair or a box/line reduction first, then a naked pair. If none is there, recheck your marks before you go looking for anything bigger.

What other mistakes do sudoku solvers make?

• Checking two houses out of three. A mark list built from the row and column alone keeps digits the box rules out. Check all three every time, in the same order.
• Leaving marks stale. A placed digit has to come out of every mark it sees. Our games do this for a right digit; a wrong one leaves the marks as they were, and on paper it is all on you. Our guide to pencil marks covers the habits that keep them honest.
• Notes mode, or not. With Notes off, a digit meant as a mark goes in as an answer, and if it is wrong it counts. Glance at the button before you tap.
• Guessing. No board on our Easy to Expert levels needs a guess, because the generator only deals boards its logic can finish. If you are stuck, a move is there; a guess that happens to work teaches you nothing about where it was.

How do you find a mistake in sudoku on paper?

1. Stop placing digits. Each new one can be built on the error and makes it harder to trace.
2. Read the sign, not the square. The board tells you something is wrong in three ways: a square with no candidates, a house with no place left for a digit, or two squares of one house that can only take the same digit. The square that shows the sign is rarely the one at fault. In our runs on Hard and Expert boards, about seven breaks in ten appeared in a row, column and box that did not hold the wrong digit.
3. Check your work against the answer. Type the original givens into the Sudoku Solver, or photograph the page with the Sudoku Scanner, and compare the solution with your grid digit by digit. The Sudoku Helper can replay the puzzle from the start one hint at a time, so you can see the first place your grid parts from it.
4. Go back to that digit and ask why you placed it. A missed mark, a box that was never checked or a pair that wasn’t one: each needs a different habit.