On July 20, 2024, about 1721 central daylight time, a Just Highlander airplane, N707KM, sustained substantial damage when it was involved in an accident near Oshkosh, Wisconsin. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the pilot, she turned from the base leg to final to land on the grass runway. She reported that, when the airplane was on short final close to the ground, the wing flaps retracted from 40° to 0° uncommanded, and the airplane started to sink. The pilot added engine power to arrest the descent; however, the airplane landed hard. The pilot taxied the airplane without further incident. A postlanding examination of the airframe revealed substantial damage to the fuselage on the right side. After the accident, the flaps were manipulated in and out of the fully extended flap position using the flap actuator handle. The flap controls were continuous and extended and retracted as designed. The flap actuator handle always remained secure in the detent. Investigators were unable to duplicate the uncommanded retraction of the flaps from 40° to 0°. Flight control continuity was confirmed from the two flight control sticks in the cockpit to the respective aileron and elevator control surfaces. Continuity was confirmed from the rudder pedals aft to the rudder control surface. The passenger provided a video of the landing that was sent to the NTSB Vehicle Recorders Division for review. The video was 1 minute and 29 seconds in length and showed a landing as described by the pilot. The video did not capture any aspect of the flap extension or retraction.