I'm sure I'm not the only one who has ever over cut a mortise when cutting mortise tenons. I really blew it on 4 mortises for a desk I'm building and figured I'd share my solution, for whatever it's worth. I was luck enough that the locations of the oversize mortises are in a part of the desk that is not totally visible (drawer fronts will mostly cover); if it were the front of the desk or something, I'd be adjusting the design and buying more lumber to accommodate.
You can see here that I totally ignored the line on the side of the piece and cut the mortise way too long.
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I cut some little blocks that were a little larger than the mortise, since my mortise cut was not perfectly flat.
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I traced the bloc, scored it with a marking knife, then used a sharp chisel to make the hole.
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Then glued in the block that I made
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I used blue tape to protect the leg while I flush cut the plug
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I then sanded. I think once I put a finish on it, you'll hardly be able to notice.
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