On March 25, 1999, about 1545 eastern standard time, a Cessna 550, a Citation II, N37MH, sustained minor damage when its landing gear collapsed while taxiing, after landing at the University Park Airport (UNV), State College, Pennsylvania. The two crew members were not injured. Visual meteorological conditions prevailed and an instrument flight rules flight plan had been filed for the flight that departed the Allegheny County Airport (AGC), Pittsburgh, Pennsylvania, about 1513. The flight was conducted under 14 CFR Part 91. According to the pilot, on March 4, 1999, the airplane's left main landing gear actuator was removed for overhaul, and a temporary replacement provided by the airplane manufacturer was installed. During subsequent landings, the pilot noted that the landing gear horn would sometimes activate "just for a second," at touchdown. On the day of the incident, maintenance personnel replaced the left main landing gear micro switch; however, during the subsequent landing at AGC, the landing gear horn again activated during touchdown. The pilot said he reported the incident to maintenance personnel, who told him the actuator was safe, and the airplane's original actuator would be returned quickly. The airplane then departed for UNV. The pilot further stated: "...We entered the downwind for runway 24 at 2,700 feet. Flaps went to approach at mid-field. Gear was lowered abeam the runway numbers. Before landing checklist was called for. Check list was completed...Normal landing, but gear horn sounded on touchdown for about one second...Deployed thrust reverses only to idle reverse. Speed brakes open and we rolled to the crosswind runway 34/16 and started to exit the runway. The gear horn started to blow and the wing went down trying to turn us left..." Examination of the airplane performed by a Federal Aviation Administration (FAA) Inspector did not reveal any abnormalities. The airplane's left main gear actuator was removed and taken to the airplane's manufacturer where it was examined under the supervision of an FAA Inspector. Column load testing of the actuator revealed, when the downlock switch indicated "locked," a compressive load of 2,300 to 3,200 lbs, caused the actuator to unlock and retract. Examination of the actuator's downlock switch revealed it was functioning and rigged properly. A radiograph taken of the actuator did not depict the intersection of the lock ring and the lock piston; however, it was possible to view the intersection of the lock piston and the ball bearing which operated the lock switch. According to the airplane manufacturer's report, "The radiograph shows the ball bearing resting only 1/2 up the ramp on the lock piston. This comparison indicates that the locking mechanism has not completed its full travel. In this position, the [downlock] switch was found to indicate locked." Disassembly of the actuator revealed the lock ring was installed upside-down. According to the airplane manufacturer's report, "...The chamfered side is against the [bearing] plate, but should be facing the chamfered side of the locking groove on the piston." Review of the actuator's maintenance records revealed it was repaired by the Cessna Wichita Citation Service Center, Wichita, Kansas, on February 7, 1999. During the repair, the actuator was dissembled and reassembled. The amount of cycles on the actuator since it was installed was not known. According to the maintenance facility that installed the actuator on the airplane, 31.7 hours of flight time had elapsed since installation of the actuator.