On February 4, 1994, about 0920 eastern standard time, a Cessna 172N, N9003E, registered to Palmetto Sales and Leasing, Inc., sustained damage to the left flap while in flight near Opa- Locka, Florida, while on a 14 CFR Part 91 public-use flight. Visual meteorological conditions prevailed at the time and a visual flight rules flight plan was filed. The aircraft received minor damage and the airline transport-rated pilot and second pilot were not injured. The flight originated at Opa-Locka, Florida, on February 4, 1994, at 0850. The aircraft was being rented by the FAA and operated by two aviation safety inspectors from the Federal Aviation Administration, Miami Flight Standards District Office. The pilot-in-command stated he is the supervisor of the General Aviation Geographic Unit and that he was giving indoctrination training to an inspector under his supervision. The pilot-in-command was in the right seat and the second pilot was in the left seat. The pilot-in-command further stated that about 30 minutes into the flight the second pilot asked the pilot-in-command to demonstrate a forced landing. Flaps were used during the approach to landing. Descending through 300 to 400 feet the second pilot told the pilot-in-command to initiate a go-around. The flaps were raised. As they climbed out the left flap was observed to be bent and the inboard end extended about 30 degrees. The outboard end was extended 10 degrees. The right flap was retracted. They leveled off at 1,500 feet and maintained an airspeed of 75 knots. They returned to the Opa-Locka Airport and landed without further incident. During taxi from landing it was noted that the left aileron was binding on the left flap. Postcrash examination of the aircraft by the two FAA inspector pilots indicated the left flap inboard roller was jammed in the track. When forces were applied to the flap during inspection the inboard roller became free and the flap retracted. Under supervision of an FAA airworthiness inspector the left flap, flap track, and flap rollers were removed from the aircraft. Photographs of the left flap inboard roller taken at this time showed the roller to have rust deposits on its surface and to have very little lubrication. The Cessna Aircraft Company maintenance manual requires inspection and lubrication of the flap rollers each 100 flight hours. The last recorded 100-hour inspection for the aircraft was on December 22, 1993. The National Transportation Safety Board was notified of the incident shortly after it occurred. The Federal Aviation Administration did not ask NTSB to conduct an investigation into the incident until March 24, 1994. When the NTSB asked that the left flap tracks and rollers be submitted for evaluation the operator reported to FAA inspectors that they had already been destroyed.