BUG+1
When every unsolved cell but one has two candidates, the odd cell out must take its third digit.
What it is
BUG stands for Bivalue Universal Grave: a state where every unsolved cell holds exactly two candidates. Such a position always has an even number of solutions — never exactly one — so a proper puzzle can never reach it. If you are one cell away, with a single cell holding three candidates, that cell must take the candidate appearing an odd number of times in its row, column and box. Otherwise the grid would collapse into the impossible state.
How to spot it
In the endgame, check whether all remaining cells are bi-value except one. If so, examine the three-candidate cell and count how often each of its candidates appears in its row, its column and its box. The one appearing three times — an odd count — is the answer.
Why it matters
BUG+1 is a one-move finisher: it places a digit immediately and usually resolves the rest of the grid. It only appears in the endgame, but when it does it saves a long chain of deductions.
Frequently asked questions
Why can a full BUG never occur?
In a grid where every unsolved cell has exactly two candidates, every digit appears an even number of times in each unit. Such configurations always allow at least two solutions, so a puzzle with a unique solution cannot reach that state.
Why does this technique have no interactive example here?
Because the position is genuinely rare: it needs exactly one cell with three candidates while every other unsolved cell has exactly two, and the puzzle must still have a unique solution. Our generator could not produce one, and we would rather explain the idea than show an invented grid.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
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