Other mistake: During the Poke-rap when the Poliwrath name is said, the Pokemon shown is actually Poliwag. Also, when the Graveler name is said, the Pokemon shown is actually Geodude.
Other mistake: Larvitar's weight is 158.7lb, but Ash can carry it in his backpack without any problems.
Continuity mistake: In both "Attack of the Prehistoric Pokémon" and "Charizard Chills" Charizard carries Ash on its back without any trouble. However, later on in "Charizard's Burning Ambitions", Ash is suddenly too heavy for Charizard to carry.
Continuity mistake: In the early episodes of the Advanced Generation series, May is portrayed as being slightly taller than Ash. As the series moves on, May is consistently portrayed as being slightly shorter than him.
Answer: There are a lot of fan theories around this, but the most comman theme is that Pikachu had another trainer before Prof. Oak, who gave him to Ash. Pikachu didn't appreciate being handed over.
Ssiscool