Thomas and the Missing Christmas Tree - S2-E26
Other mistake: In the shot of Donald, (or Douglas) Gordon and Henry, Donald has Edward's face.
Thomas Comes to Breakfast - S2-E18
Other mistake: The narrator said Thomas was feeling conceited, but Thomas had his sad face mask on instead.
Other mistake: The Breakdown Train to the left, in the opening scene, is derailed.
Answer: Breakdown trains help the engines back on track.