Hi Sean,
Thanks for the suggestion. I've wanted a similar feature many times myself when I've gotten to the end of a puzzle only to find out that I made a mistake somewhere along the way.
Version 1.3 will be wrapped up soon, so this feature won't make it into that version.
But, a new "undo to solvable" feature is planned for version 2.0. This will allow you to undo the puzzle to the last point where it was still solvable (i.e., just before the first mistake).
-David