A brain bug is any concept or feature in a fictional franchise that starts out small and insignificant but grows to ridiculous proportions. What began as something seen on one or two TV episodes or novels becomes a universal feature of the franchise down the road, often to the point of making absolutely no sense. What could initially be chalked up as an oversight of some kind becomes common practice, even though the concept is utter stupidity if one were to stop and think about it. Brain bugs are particularly offensive when they begin as a character trait of one or a few members of a particular group, race or species, and that trait is subsequently applied to every member of that group.
- Jefferies tubes: In TOS, certain repairs on the engines and transporters could only be done in a narrow access tube. In TNG and later, numerous repairs were carried out in such tubes, which comprised almost as much of the ship's volume as the regular corridors.
- Cross-species matings: In TOS, Spock was the product of the marriage of a Human and a Vulcan, and his conception apparently required significant medical intervention. In TNG and later, Humans and aliens could interbreed without medical assistance to produce fertile children, and such hybrids were surprisingly common.
- Monolithic cultures: In TNG and later Star Trek series, races which might have seemed culturally diverse when introduced quickly degenerated to revolve around a single cultural element. Examples include the Klingons (who became Space Vikings) and the Ferengi (who became fanatical anarcho-capitalists who literally worshipped commerce). Numerous one-episode alien races were also depicted as obsessively revolving around a single activity.