Brain bug

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 are the result of one or a few characters of a particular group, race or species having a particular trait, and that trait is subsequently applied to every member of that group.

For more information, see Mike Wong's essay on Brain bugs