I trimmed the LAST block for a top to which I had NO fabric left and I was in a hurry. BAD combination. Of course, I cut into other finished blocks that were laying on the cutting mat. Can't remember how I solved that one.
Then I was the only participant in a bag class (to test out a new machine) who had the button holes going the wrong direction.
My biggy is the (still unfinished) Lone Star. Instead of radiating out of the center, mine radiate into the point. I still don't know how I did that.