On January 12, 2014, about 1615 central standard time (CST), a Cessna 680 Sovereign, N531RC, experienced a left-hand elevator trim driveshaft failure. The two flight crewmembers on-board were not injured. The airplane was registered to Great Dane Financial, LLC and operated by CC Industries under the provisions of 14 Code of Federal Regulations Part 91 as a flight test. Visual meteorological conditions prevailed for the flight, which originated from Chicago Executive Airport (KPWK) in Wheeling, Illinois. The pilot reported that, while climbing to 15,000 ft mean sea level, the autopilot disengaged and the autopilot disconnect chime sounded accompanied by an "AP fail A-B" message. At this point, the flight crew decided to return to the departure airport. The pilot noted that, while trying to slow the climb, "extra" force was required to push the airplane's nose down. He attempted to apply nose-down trim but felt no change and then noted that the elevator trim tab indicator was showing full nose-down trim. He subsequently landed the airplane without incident. During post flight examinations, the operator tested the stabilizer trim though its full range of travel and no discrepancy in the system was noted. Additionally, no abnormality of the elevator trim tab indicator was noted throughout the range of stabilizer travel. The operator contacted a Cessna Aircraft Company Service Center to assist in troubleshooting the problem; it was determined the connecting pin linking the drive shaft to the left-hand elevator trim tab actuator had become dislodged. This failure of the Cessna 680 Sovereign elevator trim tab drive shaft pin occurred on two other occasions. Cessna Aircraft Company is in the process of proposing a change in the drive pin dimensions and method of securing the pin; this pending change will prevent the elevator trim tab drive pin from dislodging.