I agree with the pin idea. I use long thin pins and pin right before the seam to make sure they don't shift, even when they nest together. Last night I cut 4 WOF strips, sewed them together, and realized I had made 6.5 inch blocks and they were supposed to be 4.5. Wanted to slap myself with my ruler! Time to take a break and start over. Happens to us all. Ah, the joys of unsewing. :P