Which Pokémon anime episode was withdrawn after causing reported photosensitive reactions?

Dennō Senshi Porygon was the Pokémon anime episode withdrawn after reported photosensitive reactions.

The episode aired in Japan on December 16, 1997. A scene near the end used rapidly alternating red and blue flashes during an explosion, and reports linked the broadcast to seizures and other symptoms among some viewers.

Japanese broadcasters and Pokémon’s producers responded by suspending the anime and reviewing production practices. Dennō Senshi Porygon was never rebroadcast in Japan and was not released as a regular television episode internationally.

Although Porygon appears in the episode, the flashing sequence is associated with Pikachu’s attack. This is why the incident is often inaccurately called the “Pikachu shock” incident or blamed solely on Porygon.