This has to be one of my favourite alien contact stories. In November 1977, someone hijacked a television transmitter in the south of England, and managed to drown out the ITV news with a message of their own. The new voice claimed to be an alien. Here's his message.
Surprisingly, there are many people who believe this to be a genuine alien broadcast. No-one has ever owned up to the prank, and it is believed that some pretty sophisticated work went into perpetrating it. I doubt the culprit will own up - the television company said it intended dealing very harshly with them, as many people who heard the broadcast were terrified.