On September 23, 2006, approximately 1015 eastern daylight time, an Engineering & Research Ercoupe 415-C, N3233H, collided with a tree after experiencing a loss of power during the initial climb-out after takeoff from a private airstrip about eight miles north of Trenton, Florida. The private pilot, who was undergoing a flight review, and the certified flight instructor who was giving the flight review, received minor injuries. The aircraft, which was owned and operated by the pilot, sustained substantial damage. The local 14 CFR Part 91 proficiency evaluation flight was operating in visual meteorological conditions at the time of the accident. No flight plan had been filed, and there was no report of an ELT activation. According to both pilots, soon after takeoff, when the aircraft was about 20 feet above the ground, the engine started surging between full power and almost no power. As the aircraft began to descend, the pilot attempted to maneuver toward an open field, but before they could reach the field, one of the aircraft's wings collided with a tree, and the aircraft cartwheeled into the terrain. During a post-accident FAA-directed inspection, the engine was run extensively at speeds from idle to 2,200 rpm. It was run with the magneto switch in the BOTH position, and with the switch on each individual magneto. The engine was stopped and restarted a number of times during the test sequence, and it was run both with the carburetor heat off and in the full-on position. During the entire test sequence, the engine ran smooth and strong, with no evidence of any malfunction or anomaly. After the test run, the carburetor was disassembled and inspected, and there was no evidence of any component malfunction or any solid contamination. In further discussions with the pilot, it was determined that the aircraft has an approved Supplemental Type Certificate (STC) to allow it to be run on auto fuel. It was further determined that just prior to the flight, the pilot topped off the fuel system with auto fuel that he purchased at a local gas station and transported to the aircraft in a five-gallon jug. Although the pilot filtered the fuel through a nylon cloth to remove any solid particles while putting it in the aircraft's tanks, the fuel was not checked for water contamination prior to being introduced to the aircraft's fuel system. Although the pilot detected no water in the fuel that he drained from the fuel system low point just after topping off the tanks, he did not rock the wings or allow any time for any water introduced along with the auto fuel to flow through the system to the low point. It was his opinion, as well as the opinion of his mechanic and the FAA inspector, that there had been water contamination in the auto fuel, and that the water did not make its way through the system and reach the carburetor until just after the aircraft took off.