If every unsolved square had exactly two options, and each number appeared twice in every row, column and box it could still go in, the board would have two answers. When one square is the exception, it takes the number that breaks the tie.
123456789
123456789
1
34
39
47
5
2
79
8
6
49
8
6
3
1
47
79
5
2
7
2
5
6
9
8
3
1
4
8
6
1
2
3
9
4
7
5
49
347
39
47
8
5
6
2
1
5
47
2
1
6
47
8
9
3
2
9
8
5
4
3
1
6
7
6
5
4
8
7
1
2
3
9
3
1
7
9
2
6
5
4
8
Loses a pencil mark
The name
BUG stands for bivalue universal grave: a board where every unsolved square has two options and every number two places in each group. It can't have exactly one answer.
How to spot one
Late in a hard board, when nearly every square is down to two pencil marks, look for the one square with three.
Only for proper puzzles
This relies on the puzzle having exactly one answer. Every board in the game does; a puzzle with several answers would break it.
Practise BUG+1
Boards that need it, with the owl's hints when you're stuck.