The episode, called "Electric Soldier Porygon", is now part of Pokémon folklore. Centring around the adventures of Ash and his friends as they travel inside a Pokéball transmitter machine, its story and premise are innocent enough. The machine is broken, and the kids embark on an adorable little cyberspace adventure to fix it.
What caused all the problems were the animation techniques employed in the episode. There comes a point, around twenty minutes into the show, when Pikachu uses his lightning attack to blow up some missiles. Because these are virtual missiles, and Pikachu is currently residing in Pokémon's version of cyberspce, a regular explosion just wouldn't look right.
So the animators used a rapidly-strobing technique that flashed red and blue lights on the screen, to make the explosion look "virtual". Like something you'd see in Tron, or the Lawnmower Man.
And then all hell broke loose.
Straight away, children across Japan were struck down with various ailments. Some kids passed out, or experienced blurred vision. Others felt dizzy, or nauseous. In extreme cases, some even experienced seizures and cases of temporary blindness.
While the exact number of children legitimately affected by the show will never be known, in total 685 kids (375 girls, 310 boys) were put in ambulances suffering some kind of medical problem after watching the episode. While most made speedy recoveries - some within minutes after the show's conclusion - a small number were diagnosed with epilepsy, which had been triggered by the rapidly-blinking display.
The incident, which became known in Japan as "Pokémon Shock", was a disaster for children's animation in Japan, Pokémon and Nintendo, whose stocks took a hit. The show was taken off the air for nearly four months while its producers and health professionals scrambled to discover what had caused such a concentrated outbreak of health issues. It also resulted in a wave of negative, if ill-informed comments from the American media.
When it eventually returned, it did so with several changes. The show's opening credits had been altered to eliminate the possibility of a repeat incident, and the first episode back was preceded by an "informercial" of sorts (left), which sought to explain what had caused "Pokémon Shock" and reassure viewers that steps had been taken to ensure that it never happened again.
As a result of "Pokémon Shock", "Electric Soldier Porygon" has never been broadcast again in any region, even in edited form. And despite having nothing to do with the incident, the Pokémon most closely-associated with the show, Porygon, was never shown in the series again either.
So what caused "Pokémon Shock"? Ultimately, it was found to be a combination of the effects of strobe lighting combined with the sheer popularity of the program. It's estimated that around 1 in 4000 people are vulnerable to "photosensitive seizures" and other health issues when viewing strobe lighting. That may sound minor, but when you consider over four million kids were watching that particular episode, it's easy to see why so many were struck down.
While it's easy to look back on the incident now and laugh - as South Park famously did two years later - we also have to remember that epilepsy is a serious condition, and that while the photosensitive seizure warning messages before Nintendo (and other company's) games may be a hassle, this episode showed they're there for a very good reason.
If you'd like to see the sequence in question, you can view it in this clip, but be warned, you obviously do so at your own risk!
Total Recall is a look back at the history of video games through their characters, franchises, developers and trends.