On December 25, 1997, at 2319 Pacific standard time, a Boeing 737-522, N918UA, flight number 318, registered to and operated by United Airlines as a 14 CFR Part 121 scheduled passenger transport, was standing at the gate at the Eugene, Oregon, Airport and in the process of deplaning passengers when the Captain ordered an emergency evacuation after ground personnel reported a fire in the number 2 engine. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan had been filed for the flight. The airplane was not damaged. The two airline transport rated pilots, three flight attendants, and 100 passengers were not injured. The flight had originated from San Francisco, California, about one hour and 30 minutes before the incident. United Airlines personnel reported that the flight crew was told to expect ground power to be hooked to the airplane at the gate. The aircraft was taxied to the gate and the jetway was positioned to the 1 left door. While the passengers were beginning to deplane, the captain positioned the fuel levers to "cutoff" to shutdown both engines. The ground power had not yet been connected and the airplane went dark. The captain then positioned both fuel levers back to the "idle" position. Immediately following this action, ground crew noticed smoke and notified the flight crew of a fire in the number two engine. The captain immediately initiated an evacuation by using the PA system. The flight crew then performed the checklist procedures and when they got to the position of the fuel levers step in the list, the captain noticed that the fuel levers were in the "idle" position. The captain then positioned the fuel levers to the "cutoff" position. The smoke began to dissipate from the engine and the all clear signal was given by the ground crew. After the smoke dissipated, the captain cancelled the evacuation. Approximately 20 passengers had deplaned via the 2-right slide to the ground. Another 10 to 20 passengers went out the over wing exit. When the evacuation was cancelled, the passengers that had not reached the ground, came back inside the cabin and exited via the 1-left door. The slide at 2-left did not deploy as the door had not been re-armed. The flight attendant reported that the event was over before the door could be closed and the slide re-armed. Mechanics inspected the engine and reported that there was no indication of an actual fire and suspected that when the fuel lever was positioned to the "idle" position, the fuel valve opened and jet fuel was injected into the decelerating hot engine.