Starbase 200

From Memory Alpha, the free Star Trek reference.

Starbase 200 is a Federation starbase. In 2267, the USS Enterprise was to proceed to Starbase 200 after it had charted a planet with an iron-silica body and an oxygen-hydrogen atmosphere. Instead, the Enterprise was diverted from its course when it twice detected momentary discontinuances in local spacetime. (TOS: "The Alternative Factor")

In every other episode of the Original Series and the Animated Series in which Federation starbases are mentioned by number, every starbase number was less than 30. Starbase 200 is the only exception.