I found this through the Snopes message board. Two comments: one, I think it's cool that this guy was never found, and two, this creeps me out for some reason. I couldn't imagine seeing this live! Anyway, here's the story....
<object width="425" height="350"><param name="movie" value="http://www.youtube.com/v/OnDYssFcNxc"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/OnDYssFcNxc" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"></embed></object>During a broadcast of the Dr. Who episode "Horror of Fang Rock" on WTTW Chicago Channel 11, on Sunday November 22nd, 1987, at around 11:15pm, a Video "Pirate" wearing a Max Headroom mask broke into the signal and transmitted one of the weirdest, unauthorized things ever to hit the Chicago airwaves. The man was never caught.
Since it's such crappy quality, here's a (partial) transcript, too:
"He's a freaking nerd!"
"This guy's better than Chuck Swirsky." [another WGN sportscaster at the time]
"Oh Jesus!"
"Catch the wave..." [reference to a Coke commercial at the time of which Max Headroom was a spokesperson]
"Your love is fading..."
"I stole CBS."
"Oh, I just made a giant masterpiece printed all over the greatest world newspaper nerds." [??]
"My brother [mother?] is wearing the other one."
"It's dirty..."
"They're coming to get me..."