On November 26, 1977, the audio portion of the ITV television transmission in southern England was hijacked. Vrillion of Ashtar (audio) addressed the people of Earth (wiki) in what was widely believed to be a hoax.
Some aren't so sure.

This is the voice of Vrillon, a representative of the Ashtar Galactic Command, speaking to you. For many years you have seen us as lights in the skies. We speak to you now in peace and wisdom as we have done to your brothers and sisters all over this, your planet Earth.