Scheduled to air on the morning of December 31, 1997 in Japan, was a one-hour Pokémon New Year's special. The title (which, unlike other Pokémon episodes, is written in kanji) was "大晦日だよ！ ポケットモンスターアンコール" (translated as "It's New Year's Eve! Pocket Monsters Encore"). It was believed by many to be the intended 40th episode of the anime, although these claims have been quashed by a booklet included with the Laserdisc release of Mewtwo Strikes Back!, which listed the episodes thus far (in production order), but didn't include the New Year's special. Other citations proving this are the production numbers seen on several Pokémon episode scripts that were, at one time, available. It was instead intended to air between episodes 39 and 40 as a one-off celebratory special.
The episode was cancelled before broadcast, during the rescheduling that followed the infamous "Electric Soldier Porygon" incident in which bright pulsating lights caused nearly 700 Japanese children to be hospitalised after suffering epileptic seizures. The infamous "seizure episode" was never broadcast again in any country, but eventually surfaced online. It is considered most likely that the New Year's special was a recap episode, as all other Pokémon episodes with "Encore" in the title were, and given the fact that it was intended to air as a special ("special" episodes often being recaps); this however has not been officially confirmed.
2 years later, in 1999, a 2 hour special titled "お正月だよ！ポケットモンスタースペシャル！" aka "It's the New Year! Pocket Monsters Special" aired, covering the events of the anime up to episode 77. It's likely that the intended 1 hour 1997 special was revamped into a 2 hour special due to the quantity of new episodes that had been produced in the 2 years since the intended 1997 broadcast date.
The original 1997 episode has never been aired in any country, and no further information, trailers, or screenshots have surfaced.
This episode is 1 of 4 Pokémon episodes that have never aired, the other 3 being "Battle of the Quaking Island! Barboach VS Whiscash!!" and "Rocket-dan VS Plasma-dan!" Parts 1 and 2.