I’d never heard of this before tonight but it’s certainly interesting, I’ll quote wikipedia:

Polybius is a supposed arcade game featured in an Internet urban legend. According to the story, the Tempest-style game was released to the public in 1981, and caused its players to go insane, causing them to suffer from intense stress and horrific nightmares. A short time after its release, it supposedly disappeared without a trace. No evidence for the existence of a game, other than the ROMS (see below), has ever been discovered.

You can read the article from wikipedia here, and I’ve even found a playable reproduction of the game here.