Kikyo Gym! Sky Battle!! - S3-E15
Continuity mistake: When Pikachu agrees after Ash says, "Someday, I'll go back and see my old friend," his mouth is open, but in the next shot, it is closed. Also, Brock is to the left of Ash, but in the next shot, he is not. (00:06:32)
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 ★