Continuity mistake: When Billy pulled the medallion out of the ground, part of the cloth covering it was hanging past his hand, but when he blew dirt off the cloth, the cloth was compactly covering the medallion. Also, the cloth should have had more than some loose dirt on it and the medallion - after buried in the ground for 200 years - should have been dirty and tarnished, not clean. (A cloth covering the medallion would not protect it from soaking rains). (00:23:35)
Revealing mistake: Close-up views of Billy riding Allie on the mini-bike (trying to escape the men following in the Jeep) did not include the lower half of the mini-bike; they were probably on a platform/trailer being pulled by a car. A 12-year-old (?) boy would have difficulty steering and maneuvering a mini-bike with a rider similar in size/weight and would have undoubtedly wrecked when he turned his body around from the waist up to look in back of him at the tailing Jeep. Billy also kept turning the throttle. (00:25:40)
Continuity mistake: When Billy goes back in time and the boy king Huko unzips his vest, the medallion is hanging at the top of the zipper but was hanging much lower earlier. (The length of the rope changes). (00:31:34)
Continuity mistake: When King Cobra was wearing the medallion, the rope was different - more like leather or suede. The stone also looked green instead of blue. (00:41:10)