Here's a sad tale about what can happen when you give back blocks that are too small:
As I've mentioned more than once, my local quilting group here in my town did a boomish swap, with 12.5" blocks like we do. Our 'leader' is a sweet older lady, about 90 years old but still going strong! She loves to do these blocks "quilt as you go" style, and quickly sandwiched all of hers and quilted them. They all looked like 12.5" blocks.... But when she went to join them into rows, she had three good rows and then the 4th was 1" smaller than the other three! Those blocks were imperceptably smaller than 12.5", but put together they really came up short. I feel so sad for her because there is no good fix for this, having quilted them already. Boo hoo... measure your blocks and fix them!