A few years ago, one of our FAs, who has been studying to be a pilot, noticed an aileron full up during taxi-out while looking out a cabin window (she was a ticketed passenger riding another airline, in an MD-80 series airplane), and mentioned it to the cabin crew. The aircraft stopped, one of the pilots came back and had a look for himself. They went back to the gate. Who knows what may have happened had they gone airborne with it?