Telepathic warning buoy placed by the Melkotians, which ordered the Enterprise to turn back before the landing party was sentenced to relive the gunfight at the O.K. Corral.
The Melkotian buoy was a probe stationed at the edge of Melkotian space. In Spectre of the Gun it confronted the Enterprise as the ship approached the Melkotian homeworld on a first contact mission, broadcasting a warning to stay away.
Each member of the crew heard the message in their own native language, suggesting to Spock that the Melkotians were powerful telepaths. Kirk chose to continue on course regardless, and the landing party was then condemned by the Melkotians to a deadly recreation of Tombstone in 1881.