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)