Revealing mistake: At the beginning of the episode, the end of the set is visibly reflected of Gordon's smoke box in a close-up of Gordon.
Edward, Gordon and Henry - S1-E4
Revealing mistake: When Gordon passes a signal, the track under him wobbles.
Revealing mistake: When Gordon puffs into the ditch his face looks wonky and wires are visibly attached to his cab.
Thomas and the Breakdown Train - S1-E7
Revealing mistake: The breakdown train appears to be missing in the close-up shot of James' wheels being put back on the rails.
Thomas' Christmas Party - S1-E26
Revealing mistake: In the very last shot of the episode, loads of wires are visible in front of the engines and behind the turntable.
Revealing mistake: When Henry passes under the bridge, a tree shakes, giving the illusion that the stones were thrown. When in reality, Henry actually ran right through a low-hanging branch.
Revealing mistake: When Gordon falls into the ditch, wires under his cab are visible.
Thomas and the Breakdown Train - S1-E7
Revealing mistake: Thomas is taking on water in the opening shot, but his water cap is closed.
Answer: Breakdown trains help the engines back on track.