It doesn't happen often, but when it does, it's creepy and weird. 

I've heard of a TV station's signal being hijacked before, but here's one I somehow missed. It happened in Britain in November 1977, when someone mysteriously hijacked a newscast in progress on a network called "Southern TV," and replaced it with a warning by a supposed alien, "Vrillon, a representative of the Ashtar Galactic Command," who delivered a message to Earth to live in peace - or else. 

It lasted only a few minutes and then disappeared. All these years later, the perpetrator has never been found. 



Another famous unsolved TV interruption happened over WGN-TV in Chicago. It, too, took place in November, this time 1987, when an episode of "Doctor Who" was suddenly interrupted by some masked moron dressed as Max Headroom, who proceeded to take over the signal for several minutes with a rambling message and a very odd moment when someone whips his bare behind. 

Despite investigations by authorities, no one has ever been caught for this extremely bizarre moment in television history and it has become legendary. 



There have been a few other incidents like this, but they're very rare since the person behind it has to have the equipment and knowledge to do it - all while leaving no trace. 

The last instance of this didn't take place on terrestrial TV but rather on a national cable outlet. When HBO announced it would be scrambling its signal to defeat C-Band (aka Big Dish) satellite owners who were picking it up for free, an outraged viewer with access to satellite equipment and a knowledge of engineering, proceeded to interrupt the movie channel's signal with a warning message from "Captain Midnight." 

On April 26, 1986, he broke into a showing of the film "The Falcon and the Snowman" to express his outrage with a polite but firm anti-scrambling message. (It starts around the 1:50 minute mark on the video below.)



This time, they got the guy. He was John R. MacDougall, a disgruntled satellite dealer who watched his business fall off when HBO led the way to scrambling and had access to a satellite via his second job at a Florida uplink centre. He was traced, found, and ultimately got off pretty easy with a $5,000 fine and a year's probation. 

You can read about how he did it here.

Incredibly, MacDougall is still in the satellite business and even brags about his moment of infamy on his website.