Bulbasaur and the Hidden Village - S1-E10
Other mistake: In the scene when Team Rocket tries to steal the Pokemon, Bulbasaur saves Oddish. Ash comes to help push Bulbasaur, and for a few frames you can see 2 sets of hand prints cut out where Ash's hands go.
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 ★