In "Pikachu and Pichu," when Pikachu and the Pichu brothers come to the big play structure thing, there are a bunch of Pokemon around, including a red-and-white, ball-shaped Pokemon called Voltorb. The dubbers for the English version made a very noticeable error here: instead of Voltorb saying "Voltorb," (almost all Pokemon say parts of their names when they speak) it says "Electrode," which is the name of a similar-looking, but different Pokemon. The two are both red and white, but the difference is that Voltorb has eyes on the red part of its body, while Electrode has eyes on the white part of its body.
In the original Japanese trailer, most of the scenes were fully animated, yet not used in the final cut. Some of the shots were Ash standing on top of a crystal pillar, as the camera pans around him and he is searching for something, a bunch of random wild Pokémon running away from the crystallization, and a shot of Mewtwo standing at the entrance of a cave with lightning cracking behind it. Supposedly, the 3rd and 4th movies were to inter-connect, with Mewtwo being the one who connects them. Perhaps the anime producers decided to throw that away and instead advertise the then-new Pokémon Crystal game? After all, several elements from the game (the Unown, Suicune, Celebi) appeared in both movies.