Using coaxial cable, there was a really bad channel bleed - as shown in this video, putting a NES on channel 3 caused distortion to the static on channels 2 and 4-7.
This distortion was so bad, you could see glimpses of the game (Days of Thunder) through the static. It also made the screen shake like crazy (on a CRT).
What was the cause of this distortion? There was no wireless transmitting involved with the linked video (the NES was directly connected through coaxial, no antennas installed).
(A similar issue happens with radio)