The pilot of the Cessna 182E reported that he was conducting night proficiency takeoffs and landings. The first two takeoffs and landings occurred normally; however, after the third takeoff, just after the airplane had lifted off the runway, the pilot's seat slipped all the way to the rear stops. The pilot then pulled the throttle back to prevent a further climb, and the airplane landed hard on the runway. A subsequent examination of the seat rails, seat rail pinholes, seat adjustment pin and coil spring by a manufacturer's representative and a Federal Aviation Administration (FAA) inspector revealed no anomalies. Airworthiness Directive 87-20-03 R2 had been complied with, and an annual inspection had been performed about 7 months earlier. Another FAA inspector, about the same size of the pilot, subsequently tested the seat locking mechanisms while sitting in the seat. He found that when the seat locking pin was secured in any of the pin holes, he could not move the seat. However, when the seat pin was not securely locked in a hole, with "very little" weight shift, the seat would slide backwards to the rear stops without the pin engaging any of the pinholes.