Definitely a prewasher here but one time a batik FQ got put into stash without prewashing by accident and it bled in the quilt. Thankfully not a whole lot. I try to keep my unwashed fabric in a special bin but I must have pulled this one to see if it would work color wise from my unwashed bin and forgot and left it with the rest of the fabrics for the quilt. I scrutinize a pattern before I make my first cut, usually to see if I prefer making the unit a different way than the pattern writer says to do it (which tends to be often) but also to make sure I understand each step to make the finished block. I won't start the pattern until I clearly understand each step. I can tell by sizes given if the block will actually measure to what they say but I still only cut enough for one to make sure it fits together the way it should. I got burned once on a pattern that did not give correct cutting sizes, it was for sashing units and border units. Had way too many sashing units and the border strip was not long enough so I had to sew in a filler piece. Thankfully had enough fabric. It was the first and last quilt I ever cut everything first before starting to sew. I tend to break those steps up a bit.
Marking tools are always tested to the best of my ability. I tell my clients I mark using a water soluble marker or chalk and the quilt will need to be washed to get all the marks out. I spritz them out as I go, but only a thorough soaking will remove all the ink of the water soluble, even if it no longer shows on the quilt top the ink is often diluted and absorbed into the batting. Chalk usually brushes out. I test the water soluble in the seam allowance on client quilts. On my own, I tend to be less cautious and will use Crayola washable markers without testing if I have used the color before. They have always come out for me but if it is a color I have not tried before, I will test. Love the Crayola ultra washable markers but you can't spritz those out as you go like you can with the blue water soluble.
If the quilt is a gift, I usually hand embroider the label as I too have had micron pigma pens fade even after heat setting. Others have lasted fine so you never know. Embroidered labels won't fade to the point of barely being able to read them.
New tools or techniques, like a new type of ruler I always test and experiment with scraps. Mostly to familiarize myself with using the tool. It is sometimes a year or more between seeing the demo and buying the ruler before I actually use it. Sometimes the test unit will be used in something other times it ages in a bin to maybe get used or eventually tossed. I like Tartan's idea of doing them in a color scheme to use in a someday sampler. I am kinda/sorta doing that now with flannel test blocks. They are destined to be a back. Finally testing some of my specialty rulers. I also like Granny Cheechee's idea of using test blocks for placemats.