Ahhh. I probably have Placement Feedback turned off, because I wasn't sure what it was for.
As you know, I prefer to solve on paper, but if I mess up a puzzle (usually a large multi-grid such as a Clueless), it's often because I made a mistake with eliminating pencil marks or transferring between grids. A couple of times now, I've restarted a puzzle and then used either SudokuSolver or JSudoku (for the puzzles types SudokuSolver can't handle) to enter the solved cells and locked candidates from the first pass and check my pencil-mark clean-up, and it's helped.
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