From "Patterns of Force", a second season episode of Star Trek: The Original Series (episode #50), broadcast on February 16, 1968.

Star Trek Wiki: "Patterns of Force": "The Enterprise discovers a planet where a Federation historian has apparently interfered with its society to have it model Nazi Germany. (...) Because of its subject matter (specifically the use of Nazi uniforms and symbols in a manner banned by the German Constitution), this episode was withheld from broadcast by the German station that aired TOS, as large segments of the episode were technically illegal. Austrian state-owned TV, on the other hand, did broadcast it, although untranslated with German subtitles (translation of the other episodes was done in Germany and bought by Austrian TV stations). Southernmost Germany in range of Austrian TV broadcast thus could watch the episode. In Germany a translated version was aired in 1999, but only late at night on pay-TV (as well as a home video release)."

Wikipedia has more: "In Futurama this episode is parodied during a flashback which shows Star Trek fans taking over the Earth in an attempt to pattern it after the program. One sign identifies a country as "Nazi Planet Episode Land (Formerly Germany)".