Shortly after takeoff, the airplane was observed turning as if to return for landing. A witness reported observing it maneuvering to line up for a landing on the runway opposite its takeoff runway. The witness stated he observed the airplane wings level and moving sideways. The airplane then stalled and spun to the ground. On-scene investigation revealed that the landing flaps were fully extended. The airplane was equipped with an FAA-approved Short Takeoff or Landing (STOL) wing modification. The height of the wing flow fences were found to be incorrect according to the STC drawings. An FAA test pilot stated that he had required that the flow fences be lowered due to aerodynamic instabilities with fences too high. Other similar airplanes were found with the wing flow fences too high. Investigation into the pilot's flying habits revealed that he often performed wing's level rudder turns. The investigation also revealed a partial magneto failure which would have resulted in a partial loss of engine power.