Later that night around 11:15pm during a broadcast of the Doctor Who episode Horror of Fang Rock on WTTW, the signal was hijacked by the same person. It was the same video that was broadcast during the WGN hijack, but this time there was audio.There's footage on YouTube, natch, of the actual video (warning: it's really creepy) and of the CBS News report that I saw.
The person in the Max Headroom mask interrupted the broadcast, saying "He's a freaky nerd" before laughing and stating "This guy's better than Chuck Swirsky!". The person continued to utter strange phrases including a Coke advertising slogan (Max Headroom was a Coke spokesperson at the time), humming the theme song to Clutch Cargo (pausing midway to say "I stole CBS"), before finally undressing below the waist and was spanked by an unknown woman with a flyswatter before the masked person cut off his transmission. It was over in about 90 seconds. The pirate was never caught.
WTTW, which maintains its transmitter atop the Sears Tower, found that its engineers were unable to stop the hijacker because at the time there were no engineers on duty at the Sears Tower. Also, the station's master control center was unable to contact its transmitting equipment remotely to switch the STL (Studio To Transmitter Link), unlike their counterparts at WGN-TV, who were able to thwart the intruder by switching their John Hancock Center transmitter STL remotely within seconds.
But to be fair, the point where they go back in to Doctor Who is pretty darn funny. "Looks like a massive shock; he died instantly."
Much less mindflaying is the classic HBO "Captain Midnight" takeover, which features a test pattern and a friendly greeting from the good Captain himself. (Unlike Max Headroom and his dom, Captain Midnight was caught, fined $5000 by the FCC, and served a year's probation.)