1Definition: B.U.G. stands for Bi-value Universal Graveyard. It refers to the fact that if all of the unsolved cells are Bi-Value Cells (BVCs) and the digits of every candidate occur exactly twice in any lane or block, then the puzzle will have multiple solutions, and hence will fail the uniqueness test. The +1 refers to the saving grace: one cell that is not a BVC, and thus saves the other BVCs from the "graveyard." This cell will have one digit that occurs three times in its lane or block, and this digit will be the solution (Big Number!) for that cell.
B.U.G.+1: All unsolved cells are BVC's except F6. Its candidate 9 occurs three times in its row/col/block. So 9 is the solution to cell F6.
2Look For: A board with nothing left to solve but BVC cells, except for one cell with three candidates. In Sue, using the Tool tab's BVC button is helpful in spotting this pattern.
3Consequence: The non-BVC cell can have its candidate erased which occurs three times in its lane or block.
4Why it Works: If there were not one non-BVC cell, the puzzle would not be unique. With the non-BVC, resolving to the thrice-occuring digit causes a ripple effect of naked singles and/or full houses, leading to a quick solution to the entire puzzle.
5Taking a look at our example, and highlighting all BVCs, we see that F6 is the only unsolved cell that is not a BVC. Its candidates are 1, 7, and 9. The 1 occurs two times in its row and block. The 7 occurs two times in its row, column, and block. This leaves the 9, which occurs three times in its row, column, and house. If the cell were to resolve to 1 or 7 it would quickly result in contradictions: cells with no possible solution, or two cells with the same single possible candidate.
6In contrast, resolving F6 to a 9 leads to naked singles on F3, F9, D6, and E6, which will in turn lead to a 9 in D3 and E9, etc...
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