Wherefore Art Thou, Pokémon? - S2-E19
Corrected entry: In the recap it says that Ash won a "big victory over Pokemon Master Prima", but it was actually a crushing defeat. (00:01:08)
Corrected entry: When Meowth comments that Victreebel's Sleep Powder works well, James closes his eyes, tilts his head up, and says "Yes, we're a knockout combination." As he says this, his nose inexplicably grows to about ten times its normal length, but goes back to normal after he says the line.
Correction: It's pretty obvious that this was done on purpose for comedic effect (growing noses are usually an homage to "Pinocchio"), and thus is not really a mistake. Remember, this is a hand-drawn show, so it's not like the animators would simply overlook a very noticeable detail like this.
Correction: It was presumably done as a joke.