Sunday December 16th, 2007 marks the 10th Anniversary of the infamous episode, 'Electric Soldier Porygon'. The episode only aired once and has never been broadcast outside of Japan. The Pokemon anime went on hiatus while they tried to determine what caused nearly 700 Japanese children to suffer seizures. Since that episode, more than 500 episodes of the anime have aired and there doesn't seem to be an end to the series in the foreseeable future.

Electric Soldier Porygon - December 16, 1997 Electric Soldier Porygon is somewhat of a notorious episode of the Pokemon anime, due to the fact that on its single airing in Japan on December 16, 1997, nearly 700 Japanese children suffered seizures, vomiting, irritated eyes and other symptoms due to a flashing strobe effect. The episode was never commercially released or re-broadcast anywhere in the world due to it being banned by the Japanese government. The scene which caused the seizures was a fifteen-second section in which Pikachu used an Electric attack on some vaccine missiles. The explosion was shown as a bright flashing object which alternated rapidly between red and blue. The seizures caused by this episode caused OLM to drop the strobe effects from Pikachu's electric attacks. Also, a disclaimer is now broadcast at the beginning of television anime precautioning viewers to not sit too close to the television screen.

Source: http://bulbapedia.bulbagarden.net/wiki/EP038