Pokemon's thirty-eight episode entitled Dennō Senshi Porygon (Computer/Electric Soldier Porygon) caused seizures to at least 658 Japanese viewers when it was aired on December 16, 1997.

The seizure was caused by red and blue flashes at extremely fast rate, triggering photosensitive seizures in which the stimuli can cause altered consciousness (Wikipedia). Because of this incident, this episode was not rebroadcasted worldwide.

So, if you're watching cartoons/anime, and there are flashing lights (even if it is not red), it's better to look away from the screen until the flashing stops. =)