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From, the puzzle solver's site
BUG stands for Bi-Value Universal Grave

As of March 2010 this strategy has been deprecated. It is extremely rare considering the alternatives - and perhaps because my algorithm for finding it was too specific. I would like to document it however, since it is unusual Uniqueness strategy..

The principle behind BUG is the observation that any sudoku where all remaining cells contain just two candidates is fatally flawed. There would have been a last remaining cell with three candidates. The odd number that couldn't be paired with another cell would have to be the solution for that cell in order to prevent the bi-value 'Graveyard'.