In 1987, the signals of both WGN and WTTV in Chicago were overtaken by two brief unknown signals, both showing an unknown figure wearing a Max Headroom mask.  To this day, there are more answers than questions about these incidents:  Who did this?  How did they do it?  Why did they do it?  To this day, these brief interruptions remain one of the biggest mysteries to occur on U.S. broadcast television.

WMAQ Max Headroom Prank on Mark Giangreco - https://www.youtube.com/watch?v=KePTSntPRtM