Mewtwo Strikes Back
Even the first Pokémon movie didn’t manage to escape the turbulence set in motion by the four month hiatus. A trio of episodes feature Mewtwo in what was to be the build up to the movie, however due to the hiatus, these episodes eventually aired several months after the movie’s release.
The easiest way to fix this is to simply move Mewtwo Strikes Back, and the
pikachu special associated with it to just after EP065; the final
episode of the trio to showcase Mewtwo.
Source: Bulbapedia