One edge is flipped
All the pieces are in place but one edge shows its two colours the wrong way round. Or in the yellow-cross step you see one or three yellow edges up, a shape the method never produces.
Why turning can't fix it
Edges always flip in pairs. A single flipped edge means the cube was taken apart and an edge went back in the wrong way round.
How to fix it
- If the scanner reported this, check the stickers of each edge first.
- If the cube really is wrong, turn the top layer 45°.
- Lever the flipped edge out gently, starting from one side.
- Push it back in the other way round.
Common mistakes
- Flipping two edges by hand. That gives a cube that can be solved, but with two edges you then have to fix again. Flip only the one that's wrong.