Thomas, Percy and the Dragon - S3-E7
Revealing mistake: When Thomas asks Percy about the guard leaving the light on, Thomas' right eye jitters.
Revealing mistake: The rails to the buffers are broken as Toad leaves the siding.
Revealing mistake: When Toad switches lines to avoid Gordon, the track ends in bushes.
Revealing mistake: When Gordon puffs into the ditch his face looks wonky and wires are visibly attached to his cab.
James and the Trouble with Trees - S5-E5
Revealing mistake: In the end of the episode, you can see James doesn't have his face fitted on correctly, as the gap between it and his eyes appears to be exposed.
A Bad Day for Sir Handel - S4-E6
Revealing mistake: When Sir Handel derails, you can see a small rod sticking up out of the ground from the tracks.
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.
Revealing mistake: When Thomas collects Rocky, the points are set against him. This also happens when Percy enters the siding.
Thomas Puts the Brakes On - S12-E17
Revealing mistake: When Stanley shunts Thomas into the yard, Thomas is eyeless.
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: Some trucks are completely Off the Rails at the clay pits.
The Fogman - S6-E6
Revealing mistake: A piece of slate is already seen on the ground from a previous take before the slates hit the ground.
What's the Matter with Henry - S7-E6
Revealing mistake: In a close-up shot, Henry is very dusty, and fingerprints can be seen on his smokebox.
Revealing mistake: When Samson crosses the Vicarstown Bridge, there is no shadow effect on his face.
Revealing mistake: When Gordon falls into the ditch, wires under his cab are visible.
Revealing mistake: In a shot of Duck at the smelter's, a hole is visible on his running board.
Revealing mistake: When Toby tells the Fat Controller about the broken windmill, his driver's head appears to have been digitally added.
Wrong Road - S2-E22
Revealing mistake: Wires to power the lamps are very visible on Gordon's and Edward's buffers, and bulbs are also sticking out from both their lamps.
Revealing mistake: In a close up of Stepney at the beginning, his lamp iron has a burn mark.
Answer: Breakdown trains help the engines back on track.