In studying for my CISSP, the topic of TEMPEST came up. TEMPEST is, essentially, interpreting the electromagnetic waves given off by electrical devices to recover the data they contain. This can happen accidentally, or on purpose.
An accidental example of this happened to my neighbors in college.Darren lived directly below me. Scott lived across the hall. Darren had a cheap, no-name 486SX clone, and he lived on it. Problem was, it interfered with Scott’s TV.
One night, Scott got fed up with it and called Darren on the phone. "Get off your computer," he said.
"Can’t. I’m doing my homework."
"No you’re not, you’re playing Solitaire. Cut it out so I can watch TV."
Not only was Darren’s computer interfering with Scott’s TV, but Scott could see what Darren was doing. Not plain as day, but close enough.
The next semester, Darren traded his PC in for a slightly faster Dell, and Darren’s Dell got along just fine with Scott’s TV.
Extreme examples like this are rare, but possible. Even today.