It's happened twice to me that I pre-cut the pieces for a class, and then something else got in the way and I didn't make that quilt like they said. On one, I had a Quilt of Valor I needed to finish up and then my friend wanted me to help her with a S&W which I had taught earlier. She hadn't gotten to the class. That cut fabric is still in a plastic bag. I don't even remember what to do with it.
The last class I went to: when I saw how it was being trimmed, I decided to make my blocks bigger. Too much fudge factor. So now my sashing strips are all too short. I took those strips and made a different quilt top (Hanging Gardens). That top is almost finished, but the original top needs five more blocks yet.
I usually cut a lot of pieces for the step I'm working on and try to finish that step, then cut the next step.