Although we wash our fabrics by machine multiple times and our threads over 25 times by hand, there may be excess dye that will wash out under certain conditions. Some colors (fuchsia) will back-dye a light colored background fabric. Test the bleed factor of a thread by stitching to a test fabric and washing in hot water.
That's what happened to Debbie's cute little quilt using my hand-dyed thread. After completing the top, she attempted a light stain for the background muslin fabric by getting the fabric wet. Wetting it made the red dye in the thread run and back-dye the surrounding fabric. My apologies to Debbie. How aggravating!