Continuity mistake: When Ash asks if Pikachu is ready for his challenge at the Dewford Gym, a poster behind a plant and a light go from being absent to being present. Also, the plant changes designs between shots. (00:00:03)
Continuity mistake: When Ash asks what they know about losing, the brown linings on Brock's jacket turn orange. This also happens when Ash runs out of the Pokémon Center. (00:18:12)
Answer: There are a lot of fan theories around this, but the most common theme is that Pikachu had another trainer before Prof. Oak, who gave him to Ash. Pikachu didn't appreciate being handed over.
Ssiscool ★