My biggest (and costliest) mistake was buying ALL the fabric for a mystery quilt. Ended up trashing the unfinished top because it literally made me sick (pattern and colors were way, way, way too busy). I literally got dizzy just looking at it.Not even people in my guild wanted it for a charity quilt! With mystery quilts, I now print out the clues and wait for the reveal to see if I want to make it.
Lesser mistakes? Plenty. Biggest one is not fully documenting, with photographs, the many quilts I have given away.