The Max Headroom Incident

from the youtube page:

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 things ever to hit the Chicago airwaves.

Earlier in the evening on the same day, during the Nine O'Clock News on Channel 9 (a different channel) the Max Headroom Pirate also broke in - although it was for a much shorter time and there was no audio. He was never caught.