The very end of the solve is where most people get stuck, usually with one corner that will not sit right while everything around it is nearly done. There are four reasons it happens. Only one of them means a piece was twisted by hand, and the other three are fixed by turning.
Find the picture below that matches your cube. If none of them do, the full repair guide covers every way a cube can end up impossible.
You were twisting the top corners so yellow faces up, and now the bottom two layers, which were finished, are all mixed up again. It feels like the whole solve has been thrown away.
This is supposed to happen. The corner trick borrows pieces from the bottom layers and only puts them back once every top corner is done. Keep going and they return by themselves.
Turning the whole cube round to reach the next corner, instead of turning just the top layer. Stopping halfway through the four turns because the bottom looked broken. Or starting on a new corner before the first one showed yellow. Any of these leaves the bottom mixed up for good, and the way out is to go back a stage or two and come forward again. Nothing is lost but a few minutes.
Seeing it done helps more than reading it. The last stage clip on the watch page runs the whole thing in under a minute.
The whole cube is finished, except one corner. It is in the right place with the right three colours, but rotated: its yellow points sideways instead of up. The corner trick above turns it, but then the bottom never comes back.
This is a twisted corner, and it was done by hand. Turning always twists corners in sets that cancel out, so one corner turned on its own is something no sequence can undo.
Twist that corner back in its own spot with your fingers. It takes under a minute and nothing comes out of the cube. How to twist a corner back by hand.
The top corners have the right colour on top, or nearly, but some of them are sitting between the wrong side colours. They belong in a different corner of the top layer.
Go back one stage. Moving corners to their right seats comes before twisting them, and the twisting trick will never move a corner to a different place. Check each top corner: its three colours should match the three centres around it, even if it is turned the wrong way.
One case is different. If exactly two corners need to trade places and everything else is solved, the cube was put back together wrong at some point. A swap of just two pieces cannot be done by turning. How to swap two pieces back.
Every corner is in its right seat, but two or three of them have yellow facing sideways.
This is the normal job of the last stage, not a fault. Use the corner trick in section 1 on each of them, one at a time, turning only the top layer in between.
Way of Cube can look at your real cube and tell you. Show it the six sides through the camera. If a piece has been twisted, flipped or swapped by hand, it lights up that piece on screen and shows which way to mend it. If the cube is fine, it teaches the solve from exactly where your cube is, last stage included.
Free, no account, and the camera pictures never leave your device.
Because the corner trick borrows pieces from the bottom layers while it twists a top corner, and only gives them back once every top corner shows yellow. Keep the cube held the same way, turn only the top layer between corners, and the bottom returns on its own at the end.
Until the corner at the front right shows yellow on top. The four turns, right side down, bottom left, right side up, bottom right, need two goes for some corners and four for others. Then move on to the next corner by turning the top layer only.
If everything else is solved and one corner is turned in its own spot, someone or something twisted it by hand, often a hard turn on a loose cube. One corner on its own can never be the result of turning, so no sequence fixes it. Twist it back with your fingers instead.
Not by turning. If two corners need to trade places and every other piece is solved, the cube was taken apart or popped and rebuilt with those two in each other's places. They have to be swapped back by hand.