Sudoku FlowHow to Find a Mistake in Sudoku
A wrong digit can hide for dozens of moves. Here are five quick checks that find it fast, starting with the digits you never need to suspect.
Updated 2026-10-04
Why one wrong digit matters
A single wrong digit does more damage than it seems. Every digit you place rules that digit out for the rest of its row, its column and its box, so one mistake quietly removes the right answer from other cells. Often you only notice many moves later, when a cell runs out of options.
Check 1: Never suspect the printed digits
The digits printed with the puzzle are always right. Only the digits you filled in can be wrong, so start by taking the printed ones off your list of suspects. In our videos and in our app, printed digits are black (white in dark mode) and the digits you enter are blue.
Check 2: Look for a repeat, one digit at a time
Pick a digit, say 7, and look at every 7 on the board. No row, no column and no 3×3 box may hold two of them. Sweeping one digit across the whole grid is faster than reading every row, because your eye only has to follow one shape.
Check 3: Start with your newest digits
If the puzzle was fine a few moves ago, the mistake is probably recent. Check the last digits you placed first: was each one forced by its row, column and box, or did it only look likely?
Check 4: Read the contradiction backwards
Sometimes there is no visible repeat yet, but something is impossible: a cell with no candidates left, or a digit with no place in a row. That is a mistake announcing itself. Look at the digits that removed those last candidates; one of them is usually the culprit.
Check 5: Count each unit
When nothing else works, count. A finished row, column or box holds each digit from 1 to 9 exactly once. Going unit by unit is slow but certain, and it is the same check our Find the Mistake videos show: the unit lights up and the repeated digit stands out.
Practice
Every Find the Mistake puzzle on this site has exactly one wrong digit, and you can play each board right on its page.
