My recent DUH moment came when I was deciding to do some more strip cutting but got confused on which fabric I was using for the sashings between the blocks. I cut 16 strips WOF at 2.5" in the black fabric before I realized I was using a green batik fabric for the sashings. I put those miscuts on a hanger with leftover strips for another project but found I could use them after all cutting the down to 1.5" to go on either side of the green sashing. I'd tossed the 1" leftover thinking I wouldn't be able to use them but then realized I would be making some throw pilows where I'd need some of them to go around the embroidery so not all was lost.
I'm sure there's been a lot more DUH memonts but that's the one I can think of right now as it just happened a couple weeks ago.