Running out of fabric. 3 recent quilts, I've gotten to the end and found I didn't have enough fabric for the final borders. Now, that's been my own fault - on two of those quilts, I redesigned the borders/made it bigger at the last minute, so my original fabric quantities were off. And the third, I knew it was close, but decided to try anyway.
I'm sure I've had other big "oops", but I've carefully blanked them all from my mind. All except the first mystery quilt I made - from 5 different novelty fabrics. The top made me sea-sick to look at, and I finally pitched it as irrepairable last year.