My worst (and most recent) was cutting panels wrong. Not just one panel wrong ... but two. Had to buy more panels
My other big mistake wasn't so much a mistake as it was ignorance of what could, and could not be done - it was my very first quilt so that's the excuse I'm going with
I attempted (note that word) to "quilt" the quilt with metallic thread using machine embroidery stitches ... without the benefit of a walking foot or a metallic needle. Yes, there is a reason that one is STILL a UFO after several years.