This was a prank someone pulled in 1987 where they managed to override a broadcast of Dr. Who on WTTW with a video feed of a guy in a Max Headroom disguise babbling about the GWN, Chuck Swirsky (yes, the guy who did the play-by-play for the Raptors for several years) and various other vaguely subversive things before the feed cut out and the regular broadcast resumed.
I’d really love to witness something like this firsthand, but aside from this, Captain Midnight (which was entirely less interesting and will not be shown as a part of Vince Nitro’s Private Reserve), and a handful of porn videos playing over sporting events, it really isn’t that common. It’s a shame.