BUG means *Binary Universal Grave*: every empty cell has exactly two candidates, and each candidate appears at most twice per house — an invalid state with more than one solution.
BUG+1 is the almost-BUG: every unsolved cell is bi-value except one cell with three candidates. Uniqueness says the extra candidate in that cell must be true.
Why it works
If that extra candidate were false, the grid would become a pure BUG — typically allowing multiple solutions. A unique puzzle refuses that world.
So the extra digit is forced. Place it.
How to spot it
Quick check whenever all-but-one empty cells are bi-value.
Demonstration
A unique classic puzzle cannot fall into a Binary Universal Grave (all bi-value, multi-solution).
Check whether almost every empty cell has exactly two candidates. Exactly one cell has three — that is BUG+1. The extra digit is 9.
If 9 were false, the puzzle would fall into that deadly all-bi-value grave with more than one solution.
Place 9 in the three-candidate cell. Uniqueness demands it — only on unique classic puzzles.
Example 1
Look for an all-bi-value grid with one triple cell.
BUG+1 — the extra digit is 8.
Place 8. It prevents a non-unique Binary Universal Grave.
Example 2
Look for an all-bi-value grid with one triple cell.
BUG+1 — the extra digit is 9.
Place 9. It prevents a non-unique Binary Universal Grave.