Tier 4 · first asked for by Diabolical
BUG+1
When every unsolved cell but one holds two candidates, the odd cell takes the digit that breaks the symmetry.
Also called BUG+1, Bivalue Universal Grave.
A position where every unsolved cell has exactly two candidates, and every digit appears exactly twice in every unit, has an even number of solutions — the candidates can be swapped throughout. Since a valid puzzle has one solution, that position can never be reached.
So if you are one cell away from it — every cell bivalue except one with three candidates — the odd cell must take the candidate that appears three times in one of its units. Any other choice would leave the impossible position behind.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it places
What it proves. BUG+1: every other unsolved cell is bivalue, so R4C9 must take 3 — anything else leaves a position with an even number of solutions.
Builds on Unique Rectangle.