
Mystery at the Lighthouse - S1-E13
Revealing mistake: When first showing Ash and friends at the top of the lighthouse, Pikachu is floating in midair. (00:15:00)

Revealing mistake: When Nurse Joy is in the emergency room, a light is floating in midair without being attached to anything. Also, a left door goes from being absent to being present. (00:06:31)

Revealing mistake: When one of the staff of the Pokémon League goes over to type on the keyboard, she clips through a device in the process. (00:02:40)
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 ★