BUG Type 2
BUG Type 2 extends the Bi-value Universal Grave to two escape cells: when both carry the same extra candidate, that digit must appear in at least one of them, so it can be removed from any other cell that sees both.
What is BUG Type 2?
A valid Sudoku has exactly one solution. Near the end of a solve, the grid can reach a state where almost every empty cell has exactly two candidates. If every remaining cell were bivalue and every candidate appeared exactly twice in each row, column, and block where it occurs, the grid would have two interchangeable solutions — a deadly pattern called a Bi-value Universal Grave, or BUG. Any cell that breaks this symmetry by carrying a third candidate is called an escape cell.
Type 2 is the case where two escape cells break the pattern, and both carry the same extra candidate. Because removing that shared candidate from both cells would restore the deadly BUG, at least one of the two escape cells must hold it. Any other cell that sees both escape cells cannot also contain that candidate, so it is removed from those common peers. The elimination is identical in spirit to a locked-candidate move, but it is driven entirely by the uniqueness guarantee.
How to spot it
BUG Type 2 appears late in a solve, when the grid is nearly complete and most empty cells are bivalue. Look for two cells that each have three candidates while every other unsolved cell has exactly two. If both tri-value cells share the same extra candidate, check whether each candidate digit appears exactly twice in every row, column, and block where it shows up — if so, the grid is one step away from a BUG, and the two cells are the escape pair. The shared candidate can then be removed from any cell that sees both escape cells. If the two escape cells carry different extra candidates, you are looking at Type 3 or Type 4 instead.
The logic
If the shared extra candidate were removed from both escape cells, every other empty cell would be bivalue and every candidate would appear exactly twice in each house — the BUG state, which has two interchangeable solutions. A puzzle with a unique solution cannot contain a BUG, so at least one of the two escape cells must hold the shared candidate. Any peer that sees both escape cells is therefore blocked from holding it, and the candidate can be removed from those peers. The deduction follows directly from the uniqueness guarantee without needing to know which escape cell is correct.