OOPS!!
My favorite oops is a LAQ who had a gorgeous quilt in a show. I turned it over to see the back (we all had gloves to encourage this) and on the back were some scraps that accidently got sewn into the quilt because she did it on her regular machine and did not clean her sewing area. She said it became a design element :).
I always think of this when I do an OOPS -- it helps to remember that all of us make mistakes.