Revealing mistake: When Thomas leaves the sheds, his driver doesn't have a face.
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.
Revealing mistake: When Gordon falls into the ditch, wires under his cab are visible.
Answer: He was in the water.