Solving Techniques 12
BUG+1
(Binary Universal Grave + 1)

BUG situation

Please look at the diagram below. [12][12] are in rows B and D.

If [1] is in [R2C1], the result is as follows.

If [2] is in [R2C2], the result is as follows.

In this strong link, two results can happen and both could work as the solution.
This means that there are two possible solutions to the puzzle. A Sudoku problem can’t have two solutions. This is what is known as the BUG state.

BUG+1

Below, there is the BUG from above, however, with one cell with three candidates [123]. (It is a plus one to the BUG state and hence is called BUG+1) If [1] or [2] goes into these cells, it will be in the unsolvable state like before, so in this case a [3] is entered here.

In this way, if there is a candidate that can get you out of a BUG state, using that number instead of the others, is called the BUG+1 technique.
There are more complex versions of the BUG state. Whenever there are the same two numbers to choose that, form a strong link, it is a BUG state.

More elaborate BUGs

When looking up BUGs, a crazy sample was found posted in the comment section by someone on http://www.sudokuwiki.org/BUG, so we examined it.

If the red cell is a [1]

If the red cell is a [4]

If the red cell is an [8]

Astonishingly, in each situation the solution works. It’s amazing what some people find!

Strong link

As shown below, a link where only two numbers are possible in a row, column or box, is called a strong link. In the diagram, it is between [1] and [2].

Names of cells in Sudoku

R1C1	R1C2	R1C3	R1C4	R1C5	R1C6	R1C7	R1C8	R1C9
R2C1	R2C2	R2C3	R2C4	R2C5	R2C6	R2C7	R2C8	R2C9
R3C1	R3C2	R3C3	R3C4	R3C5	R3C6	R3C7	R3C8	R3C9
R4C1	R4C2	R4C3	R4C4	R4C5	R4C6	R4C7	R4C8	R4C9
R5C1	R5C2	R5C3	R5C4	R5C5	R5C6	R5C7	R5C8	R5C9
R6C1	R6C2	R6C3	R6C4	R6C5	R6C6	R6C7	R6C8	R6C9
R7C1	R7C2	R7C3	R7C4	R7C5	R7C6	R7C7	R7C8	R7C9
R8C1	R8C2	R8C3	R8C4	R8C5	R8C6	R8C7	R8C8	R8C9
R9C1	R9C2	R9C3	R9C4	R9C5	R9C6	R9C7	R9C8	R9C9