Pokémon! I Choose You! (found original Japanese airing of anime series pilot; 1997)
Screenshot of the original 1997 airing.
Date found: 2 May 2015
Found by: Pokémon Peru
When Pokémon finally went back on air in April 1998, four months after the infamous "Electric Soldier Porygon" incident, OLM edited the previous 37 episodes, slowed down the flashing lights, and never aired the original versions again despite no previous reports of the episodes causing seizures for anyone.
Eventually, the episodes from their original airings were available on Hulu Japan, which were ripped by subbing group #pokemon-originals on April 2, 2013. Even then, however, episode one's original cut was not uploaded because Hulu replaced it with a new version that included advertisements for the Wii U before the episodes were put on the #pokemon-originals website. Because of that, the episode was still missing until May 2nd, 2015 when a fan group called Pokémon Peru once again uploaded the original cut in two parts.