I don’t recall seeing this bug mentioned in a thread before. I granted an undo in this teaching game, Furda vs. Conrad Melville. Consequently, the stones in the Ladder Variation, the basis for my suggestion of an undo, have all moved up one position. The variation is shown as being a response to 18 instead of 19, and 20 became 19, etc., resulting in a reversal of colors. It makes a complete hash of the rationale and might confuse anyone looking at the game subsequently.
To solve this, it seems like it would be possible to give a special treatment to the undone move (such as parentheses or a special color). The variation could still be attached to the undone move, followed by the new move represented in a normal way: 18, (19), 19.
