On February 8, 2001, about 1850 hours Pacific standard time, a Piper PA-28-181, N8122G, impacted a berm during a forced landing at the Montgomery Field airport, San Diego, California. Visual meteorological conditions prevailed during the nighttime flight, and no flight plan was filed. The airplane was operated by Plus One Flyers, Inc., located in San Diego, and was substantially damaged. The commercial certificated pilot was not injured. The personal flight was performed under 14 CFR Part 91, and originated from Montgomery Field about 1830. According to the pilot, he was practicing takeoffs and landings, and he checked the fuel quantity in the airplane's fuel tanks while flying on the downwind leg of the traffic pattern to runway 28R. He then repositioned the fuel tank selector to what he believed was the left fuel tank. He stated that he "inadvertently" switched the selector to the off position, and all engine power was lost when the airplane was on the base leg. The pilot additionally indicated that he immediately turned toward the airport. Airport operations personnel reported that the airplane impacted a berm on the uneven soft terrain. The airplane came to rest about 1,800 feet short of the runway's threshold. An examination of the airplane revealed that fuel was present in both wing tanks. The airport operations personnel estimated that both tanks were at least 1/2 full of fuel.