Tomorrow is Yesterday - S1-E20
Corrected entry: Why do Sulu and Kirk beam down into the Air Force base in standard Starfleet uniforms? On later such missions (Assignment: Earth - Patterns of Force) they have suitable clothing made up, but here they just stand out like sore thumbs.
Correction: In the other episodes mentioned, they were trying to blend in with the people on the planet. In this episode, it was a covert mission. They did not plan to see anyone so what they wore was unimportant.
They didn't know if they were going to run into anybody, so why not play it safe and dress appropriately.
hifijohn