Bixor

Bixor was a planet to which Duotheros sent the Sixth Doctor in order to find out its fate. It was destroyed because a worker creating a pair of trousers was distracted by the materialisation of the Doctor's TARDIS, causing a butterfly effect which lead to the planet's destruction. The incorrectly made trousers were purchased, and one day while the buyer was wearing them, the trousers fell down while in a crowded street. When people stopped to laugh at him, a pickpocket stole keys from the pocket of one of the laughing people. Upon locating the car the keys went to, the pickpocket gave away his bicycle to a nearby kid. The boy tried out his new bike, which was also the first bike he ever owned. His unsteady attempt at riding the bicycle caused him to go out onto the road, in the path of a large truck. The truck swerved to avoid the boy, and crashed into a substation which caused a power outage of an entire manufacturing sector. Once the power outage was repaired, the machinery in one of the affected facilities reactivated, causing sixteen vector modules to receive double printed circuits. The modules were installed in an aerial transport drone's guidance system. The drone was responsible for transporting dangerous materials to the capital's central nuclear power plant. The guidance system malfunctioned, just as the drone was passing over Bixor's only active volcanic fissure. The drone dived as fast as it could into the fissure, causing a chain reaction that destroyed the planet.

Upon learning that his own arrival had caused the destruction of the planet, the Doctor travelled back in time and planted a tree that would be fully grown by the time of his past self's arrival, blocking the view the worker would have had of the Doctor's arrival, meaning the worker was never distracted and made the trousers correctly. However, the Doctor noticed that other workers in a nearby building had noticed his most recent arrival. Not taking any chances, he travelled back one more day, and during the night, set up a series of mirrors to prevent his second arrival from being seen. With the changes he made, the planet was never destroyed, and only the Doctor retained the memory that it had ever been destroyed. (AUDIO: The Wings of a Butterfly)