I quilted quite a ways on a white quilt until I realized that I had forgotten to change the bobbin and was using black thread from my last quilt. Of course, it all had to come out.
I've also quilted scraps of fabric into the back of a quilt. Grrr....And, flipped the backing of an entire side of feathered border over so it was sewn onto itself. What can I say? I learn by my mistakes. (Usually)
Watson