Revealing mistake: In the first close-up of the policeman, the camera wobbles.
Revealing mistake: When Gordon is being pulled from the pond, in the close up scene of Gordon reversing up the sleeper bed, the front of his buffer beam/running board catches on the upright stake to the right hand side of the worker figure, pulling it back before it rights itself. (easier to see on widescreen version). (00:04:10)
Revealing mistake: When Gordon falls into the ditch, wires under his cab are visible.
Revealing mistake: When the boys sing, the camera shakes.
Revealing mistake: When Gordon puffs into the ditch his face looks wonky and wires are visibly attached to his cab.
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.
Thomas, Percy and the Coal - S2-E1
Revealing mistake: In some shots of Thomas and Percy, both engines have wonky eyes.
Thomas, Percy and the Coal - S2-E1
Revealing mistake: When Toby pulls Annie and Clarabel, the latter coach has a loose roof.
Revealing mistake: After Gordon stops on the bridge for the cow, his wheels keep moving for 3 seconds.
Revealing mistake: When Percy stops on the bridge, white glue is on his middle lamp iron.
Revealing mistake: Edward has wonky eyes in some scenes.
Revealing mistake: When Edward is at the last station, his whistle is crooked.
Revealing mistake: In a close up of Percy's driver, he is in Henry's cab.
Revealing mistake: When Percy is at the "backing" signal, his cylinder is sagging.
The Runaway - S2-E10
Revealing mistake: When Thomas' fireman is ill, you can see a brief flash of red outside, meaning it's James. (Note: same footage from Old Iron).
The Runaway - S2-E10
Revealing mistake: When Duck comes to Terence's field, the end of the track leads right into the bushes right behind duck.
The Runaway - S2-E10
Revealing mistake: When Duck is happy at the beginning, the breakdown train's buffer on the right is broken.
Revealing mistake: When Duck goes off to fetch Gordon's coaches following his chat with Diesel, the device beneath the tracks used to produce steam continues to smoke long after Duck has left.
Revealing mistake: Most of the engines' eyes are wonky when Diesel says their names and their eyes are wonky in a few shots of them.
Wrong Road - S2-E22
Revealing mistake: When Gordon says, "I am Gordon! Stop! Stop!", his eyes are wonky.
Answer: Breakdown trains help the engines back on track.