On December 16th of 1997, 685 children across Japan were rushed to the hospital due to "Pokémon Shock." The 38th episode of the Pokémon anime contained flashing visuals that caused many kids to suffer photosensitive epileptic seizures. This tragedy resulted in two children being hospitalized for over two weeks. The episode was pulled immediately, and it hasn't aired in any country since.
For more information, read this 2015 article from Kotaku.
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 (pictured above), 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.
Don't tell anyone I told you, but the dangerous scene can be found online if you are willing to take the risk.