According to the certificated flight instructor (CFI) and the certificated airline transport pilot (ATP), the ATP was seated in the front seat receiving dual instruction to "re-qualify" in the tailwheel-equipped airplane. Previously that day, the front seat pilot had performed two uneventful takeoffs and landings. The accident flight departed runway 25 with winds estimated to be from 270 degrees to 290 degrees at 5 to 8 mph, and remained in the traffic pattern. While on the downwind leg abeam the runway, the landing gear were extended, and the flaps were extended to 20 degrees. The front seat pilot turned the airplane from base leg to final, and with full flaps extended after crossing the threshold, reduced power and attained a three-point landing attitude. The landing was "smooth and on-centerline," but at 50 knots, the front seat pilot felt a "bump and shimmy," while the CFI felt a "slight bump, much like a skip, followed immediately by the 'feeling' of a very slight yaw of the tail to the left." The CFI took control of the airplane and applied full left rudder and brake, but the airplane did not respond. The airplane departed the right side of the runway, and the left wing contacted the ground, resulting in substantial damage to it. Post-accident inspection and operational testing of the tail wheel lock and unlock assembly revealed no mechanical anomalies. No elongation of the hole for the locking or unlocking was noted, and visual inspection of the pin revealed no deformation. The cable and pulley system for the lock and unlock was found to operate normally. Additional inspection of the airplane, its brake system, and tailwheel assembly by a mechanic with a reported 30 years' experience with the airplane also revealed no preexisting mechanical issues.