On March 14, 2001, at 1010 hours Pacific standard time, a Piper PA-23 twin-engine airplane, N333PE, impacted terrain during a loss of control after a door opened in flight near Anza, California. The airplane sustained substantial damage and the private pilot, who was the sole occupant, received minor injuries. The airplane was operated by the pilot/owner under the provisions of 14 CFR Part 91 as a personal flight when the accident occurred. The flight had originated in Fallbrook, California, about 0950, and was destined for Fort Mohave, Arizona. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The pilot reported that he had conducted a preflight inspection and performed a normal departure and climb. Upon leveling out at 9,500 feet, while at approximately 150 knots, the cabin door opened. The pilot stated that the airplane started a turn to the right and entered a descent. He added that the ailerons and rudder were not effective at controlling the airplane. The airplane continued its descent in what the pilot called a "flat spin," and when ground impact was imminent, he shutdown the engines and feathered the propellers. The pilot reported that the airplane "pancaked" into a field and he walked away to find medical attention. The pilot reported to Federal Aviation Administration (FAA) inspectors that he had his autopilot engaged when the event took place. He could not remember if he disconnected the autopilot or if it disengaged itself. The pilot added that he did not remember changing the aircraft trim settings once the door opened. According to the FAA inspectors who responded to the accident site, the pitch trim was found in the full nose up position. Examination of the aircraft revealed that pilot's door could not close and properly lock due to improperly installed upholstery. The FAA inspectors reported that a new upholstery installation left a welt of cushion near the door lock and prevented the lock from securing all the way. Two FAA inspectors attempted closing the door with one inspector seated inside the airplane pulling on the door, and the other inspector on the outside of the airplane pushing on the door. Attempts to close and lock the door were unsuccessful. There was no maintenance entry in the aircraft logbook concerning the new upholstery installation. The pilot/owner admitted to not getting a certificated mechanic to inspect and sign off on the upholstery modification.