Pokémon "Computer Warrior Porygon" (lost unaired English dub of anime episode; existence unconfirmed; 1998)
On December 16, 1997 in Japan, aired what can now be said as perhaps the most infamous anime episode of all time. The season 1 Pokémon episode, "Dennō Senshi Porygon" (translated to "Computer Warrior Porygon", but more commonly known as "Electric Soldier Porygon") involved Ash (or Satoshi, as he is known in the Japanese dub) and the gang going (digitally) inside an out-of-order Poké Ball transmitting device to determine the cause of its malfunctioning.
A little more than halfway through the episode, there are several scenes in which "vaccine missiles" are set upon the group, resulting in several explosions depicted by quick, bright, alternating flashes of red and blue in the background (and sometimes foreground). This flashing caused an estimated 685 Japanese children to be hospitalised after suffering epileptic seizures, two of whom remained hospitalised for upwards of two weeks. The event caused a huge amount of controversy, and received worldwide media coverage, sending Pokémon into a 4 month hiatus. The episode was immediately banned from reairing worldwide.
In an online Q&A with the late voice actress Maddie Blaustein (who voiced Meowth in the English dub), she confirmed that 4Kids had indeed produced and completed an English dubbed version of the episode, (with the seizure-inducing flashes assumingly edited out), however, due to the previously imposed ban on the episode, it was still not allowed to go to air (and as such has never been heard/seen by the public). The Japanese dub of the episode, that only aired once, was captured by several home viewers, and has subsequently found its way to the internet.