As for my worst oops - that is difficult as there have been so many. I totally messed up on Billi's indigo directional material on a side swap and went to three LQS to find an indigo to use. And then there was the one I made for Janice that I cried over - tried making a hexie block but the material didn't let me - oh my what a mess.
I have found that any interruptions while cutting and sewing usually means an oops - at least that is what I blame it on!