How it works
BUG is a late-puzzle pattern. As placements pile up, candidates shrink. If the grid ever reaches a state where every empty cell has exactly two candidates and every digit appears exactly twice in its row, column and box, the puzzle has two solutions (you can swap digit pairs everywhere).
Since valid sudoku has one solution, that state is impossible. The cell that prevents it (the one cell with three candidates instead of two) must take the digit that, if removed, would create the impossible state.
In practice: scan the trivalue cell. The digit that appears three times in the row, column or box of that cell is the one that has to go there.
When to look for it
Near the end of evil puzzles when almost every remaining cell is a bivalue pair.
Tips for spotting the pattern
- BUG only fires when the grid is almost solved. Most cells are already placed.
- If you see exactly one trivalue cell among many bivalues, check for BUG immediately.
- BUG is a satisfying technique because it ends the puzzle in a single placement.
Common mistakes
- Applying BUG before the pattern is fully formed. All other cells must be exactly bivalue.
- Picking the wrong digit. It is the digit that appears three times in the trivalue cell's row, column or box.
- Using BUG on puzzles that allow multiple solutions; the technique relies on uniqueness.