Pause before the error spreads
When a row, column or box appears to contain a repeated digit, resist the urge to erase several cells at once. Stop placing new digits and identify the exact unit where the conflict first became visible. A contradiction is evidence that an earlier move was unsupported; it is not a reason to make another guess.
If you are using Fun IQ’s progress check, treat an error message as a cue to inspect the most recent sequence of placements. It does not tell you which logic step was weak, so the aim is to reconstruct your own reasoning rather than repeatedly changing cells until the warning disappears.
Work backwards through confirmed entries
Start with the newest full-size digit in the affected row, column or box. Ask what proved that entry at the moment you placed it. A sound answer names a specific reason: it was the only remaining candidate in that cell, or the digit had only one legal position in the unit.
If you cannot state the reason without referring to a later placement, change that digit back into a candidate note. Continue backwards only as far as necessary. Earlier placements with a clear explanation can remain; a recovery is more useful when it preserves the parts of the solve that were already valid.
Rebuild the local candidates
After removing the first unsupported digit, inspect its row, column and 3×3 box. Rewrite only the candidates affected by that change, using the digits that are absent from all three units. Do not copy an old note automatically: a candidate list is a snapshot of the current grid, not a permanent label.
Keep notes deliberately small. If a cell has many possibilities, it may be better to leave it blank and return to scanning. The goal is to reveal a forced placement, not to fill every corner with marks that make the next contradiction harder to read.
Use the contradiction to find a better question
Look at the rejected placement and ask which assumption made it look convincing. Perhaps a candidate was not removed after a nearby digit changed, or a pair was applied outside its shared row, column or box. Naming that mistake gives you a specific check for the next few moves.
For example, a naked pair only removes its two candidates from other cells in the exact unit that proves the pair. If the pair came from one row, it does not automatically affect a neighbouring row. Rechecking the unit boundary is often faster than searching the whole grid again.
Resume with a small logical loop
Restart with one short cycle: scan a row, column and box around the corrected cell; place a digit only when you can state the exclusion; then rescan the three units changed by that placement. This local loop is less dramatic than hunting for a clever technique, but it rebuilds confidence in the grid one checkable step at a time.
If nothing is forced after a careful pass, move to a different part of the grid or take a break. A pause is preferable to entering a speculative digit. Sudoku progress comes from preserving valid possibilities until the grid supplies enough information to eliminate one.
Keep a lightweight error record
For a paper puzzle, place a small dot beside the first unsupported entry before erasing it. For an online puzzle, make a quick note such as ‘assumed a single before checking the box’. The record should describe the method, not judge the result. Over time it highlights whether errors usually come from stale candidates, missed duplicates or rushed pair eliminations.
Use the note once, then return to play. Fun IQ’s guides to scanning, candidate notes and naked pairs offer focused next steps when a particular part of the recovery keeps recurring. The related links beneath this guide point to each method.