Happenend to me once, in a C182, during type rating instruction. On a go-around, the left flap did not retract. Plane rolled to the right, but at slow speed, I could just level it with full left aileron and some rudder. The instructor, seeing my predicament, took a look back and identified the problem. Then we hit the flap selector down and the right flap extended. The plane was light, so we could climb, make a full flaps circuit and land normally. Cause: A screwdriver left in the wing after maintenance got stuck in the flap gear. Had some guardian angels that day. And had some animated discussion with the maintenance guy after the hands stopped shaking.