There are many great, memorable episodes (and movies!) from the classic Pokémon anime series, which follows the adventures of the young Pokémon trainer Ash Ketchum and his friends Brock and Misty, but unfortunately, there is one episode that is memorable for all the wrong reasons. Early in the series' initial run in its native Japan, just as the show had already become immensely popular, one episode caused a massive medical event for the children who watched it, causing the episode to be banned forever.

The 38th episode of the first season, "Dennō Senshi Porygon", only aired once, on December 16, 1997, because it contained flashing red and blue lights that caused hundreds of children to have strong physical reactions, including seizures. According to the retrospect of the event from The Guardian685 children were taken by ambulance to hospitals, and about 12,000 reported symptoms over the next few days. While epileptic seizures caused by flashing lights contributed to the figure, it was only a small percentage, and the mass reaction puzzled doctors and scientists for years. However, immediately afterwards, the main goal was to prevent something similar from happening again. So what actually happened, and how did the world react?

In the episode, the Electric-type Pokémon Pikachu uses an attack against computer viruses in a digital world that cause red and blue flashes. While Pokemon has used intermittent flashing before, it has never caused this kind of mass reaction. A day after the show aired, the station apologized and suspended production and broadcast pending an investigation. The series was pulled from rental shelves and Japan's Ministry of Health and Welfare even held an emergency meeting to discuss what to do. Since the specific colors, length and type of flashing appeared to be at least partly to blame, it was deemed safe to return the show to the air about four months later. TV Tokyo will put warning labels on all episodes, similar to the photosensitivity warnings that appeared (and still appear) before many video games. Episode 38 will never air again and they wouldn't even be dubbed into English for a cut version when the show began airing in syndication in the US a year later.

When the series returned to Japan after its hiatus, an explanation for the children was released at the beginning of the episode to help them understand what had happened and to reassure them that it would not happen again. The Pokemon Panic, as it was called, still has no official explanation, although many believe it was a combination of a small number of children having seizures, while many others suffered from the effects of mass hysteria. While there have been other, more limited cases of people having seizures due to photosensitivity, there hasn't been anything quite like Pokemon Panic before or since.