Gathering the Gang of Four! - S9-E44
Continuity mistake: When Brock says that he is wondering what Officer Jenny will be doing after work, the green collar on his shirt turns skin-colour, making it appear as though his head is disconnected from his body. (00:18:27)
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 ★