Some time a few years ago, some folks on the security team were swapping crackpots stories, and this one somehow lodged in my mind. The story below is paraphrased, but the essense remains intact.
"We had a crazy guy call into the security support line many years ago. He was going through a messy divorce and was convinced that his wife's lawyers were hacking into his computer in order to frame him, thereby gaining the upper hand during divorce proceedings. He wanted to know how to protect his computer against them. We played along and told him to keep the computer unplugged from the Internet, and he laughed a triumphant laugh and said condescendingly, 'They're coming in through the power lines, man! Don't you know anything?' Now, yeah, today we do have Internet access over the power lines, but still, that guy was crazy!"